Semiconductor device and method of fabricating the same

ABSTRACT

The purpose of the present invention is to provide a reliable semiconductor device comprising TFTs having a large area integrated circuit with low wiring resistance. One of the features of the present invention is that an LDD region including a region which overlaps with a gate electrode and a region which does not overlap with the gate electrode is provided in one TFT. Another feature of the present invention is that gate electrode comprises a first conductive layer and a second conductive layer and portion of the gate wiring has a clad structure comprising the first conductive layer and the second conductive layer with a low resistance layer interposed therebetween

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device including acircuit comprising a thin film transistor (hereinafter referred to as aTFT) over a substrate having an insulating surface and a method offabricating the same. Particularly, the invention relates to a structureof an electro-optical device typified by a liquid crystal displaydevice, and an electronic equipment incorporating the electro-opticaldevice, or relates to a structure of an electro-optical device typifiedby an EL (electro luminescence) display device using an EL materialcapable of obtaining electro luminescence, and an electronic equipmentincorporating the electro-optical device.

Incidentally, in the present specification, the term “semiconductordevice” indicates any devices functioning by using semiconductorcharacteristics, and includes the foregoing electro-optical device andthe electronic equipment incorporating the electro-optical device in itscategory.

2. Description of the Related Art

A semiconductor device including a large area integrated circuit made ofTFTs is under development. An active matrix type liquid crystal displaydevice, an EL display device, and a contact type image sensor are itstypical examples.

The TFT can be classified according to its structure and fabricatingmethod. Particularly, since a TFT (crystalline TFT) including asemiconductor film having crystal structure as an active layer has ahigh field effect mobility, it has been possible to form variousfunctional circuits.

In the present specification, the semiconductor film having the crystalstructure includes a single crystal semiconductor, a polycrystalsemiconductor, and a microcrystal semiconductor, and further, includes asemiconductor disclosed in Japanese Patent Application Laid-open No.Hei. 7-130652, No. Hei. 8-78329, No. Hei. 10-135468, No. Hei. 10-247735,or No. Hei. 10-135469. The disclosure of the above Japanese PatentApplications is incorporated with herein by reference.

In an active matrix type liquid crystal display device, for everyfunctional block, a pixel region (also called a pixel matrix circuit)comprising n-channel TFTs and a driver circuit including a CMOS circuitas a basic unit, such as a shift register circuit, a level shiftercircuit, a buffer circuit, and a sampling circuit, are formed over onesubstrate.

In the contact type image sensor, an integrated circuit such as asample-and-hold circuit, a shift register circuit, and a multiplexercircuit is formed by using TFTs.

Since these circuits do not necessarily have the same operationcondition, characteristics required for TFTs have been naturallydifferent not a little from one another.

The characteristics of a field effect transistor such as a TFT can beconsidered by dividing them into a linear region where a drain currentincreases in proportion to a drain voltage, a saturation region whereeven if a drain voltage is increased, a drain current is saturated, anda cut-off region where even if a drain voltage is applied, a currentdoes not flow ideally. In the present specification, the linear regionand the saturation region are referred to as an ON region of a TFT, andthe cut-off region is referred to as an OFF region. For convenience, adrain current in the ON region is referred to as an ON current, and acurrent in the OFF region is referred to as an OFF current.

A pixel portion comprises a switching element made of an n-channel TFT(hereinafter referred to as a pixel TFT) and an auxiliary holdingcapacitance, and applies a voltage to a liquid crystal to drive it.Here, the liquid crystal is required to be driven by an alternatingcurrent, and a system called frame inversion driving has been adopted.Thus, as characteristics of TFTs to be required, it has been necessarythat the OFF current is sufficiently reduced.

Since a buffer circuit of a driver circuit is applied with a highdriving voltage, it has been necessary to increase withstand voltage.Besides, in order to increase current driving performance, it has beennecessary to sufficiently secure the ON current.

However, there has been a problem that the OFF current of a crystallineTFT is apt to become high. The crystalline TFT has been regarded asbeing inferior to a MOS transistor (transistor fabricated on a singlecrystal semiconductor substrate) used for an LSI or the like inreliability. For example, a deterioration phenomenon such as a loweringof ON current has been sometimes observed in the crystalline TFT. It hasbeen considered that this cause is a hot carrier effect, and hotcarriers generated by a high electric field in the vicinity of a draincause the deterioration phenomenon.

As a structure of a TFT, a low concentration drain (LDD: Lightly DopedDrain) structure has been known. In this structure, an impurity regionhaving a low concentration is provided between a channel forming regionand a source region or drain region added with an impurity at a highconcentration, and this low concentration impurity region is called anLDD region.

According to positional relation to a gate electrode, the LDD structureincludes a GOLD (Gate-drain Overlapped LDD) structure where it overlapswith the gate electrode, an LDD structure where it does not overlap withthe gate electrode, and the like. The GOLD structure has been able torelieve a high electric field in the vicinity of a drain, to prevent thehot carrier effect, and to improve the reliability. For example, in“Mutsuko Hatano, Hajime Akimoto and Takeshi Sakai, IDEM 97 TECHNICALDIGEST, p 523-526, 1997”, it is ascertained that extremely excellentreliability can be obtained in the GOLD structure of a side wallcomprising silicon as compared with TFTs of other structures.

On the other hand, as another problem in relation to a large areaintegrated circuit, there has been a problem of wiring. An integratedcircuit comprising TFTs is provided with a gate wiring line connected toa gate electrode and a data wiring line connected to a source electrodeor drain electrode. Particularly, the gate wiring line has a problem ofa wiring delay due to influence of parasitic capacitance and wiringresistance. Although a material such as molybdenum (Mo), tantalum (Ta),or tungsten (W) has been used for the gate electrode and the gate wiringline in view of heat resistance, these have a sheet resistivity of about10 U, and have not been suitable for a large area integrated circuit. Ithas been originally preferable to use a low resistance material such asaluminum (Al) or copper (Cu).

However, the GOLD structure has a problem that the OFF current becomeshigh as compared with a normal LDD structure, and it has not beennecessarily preferable to form all TFTs with the GOLD structure in alarge area integrated circuit. For example, in a pixel TFT, if the OFFcurrent is increased, a power consumption is increased and anabnormality appears on image display. Thus, it has not been preferableto apply a crystalline TFT of the GOLD structure as it is.

Moreover, the LDD structure has a problem that the ON current isdecreased by an increase of series resistance. Although the ON currentcan be freely designed through a channel width of a TFT and the like,for example, it has not been always necessary to provide the LDDstructure in the TFT constituting a buffer circuit.

SUMMARY OF THE INVENTION

The present invention has a problem of providing a TFT with an optimumstructure for every functional circuit in a semiconductor deviceincluding a large area integrated circuit typified by an active matrixtype liquid crystal display device, an EL display device, and an imagesensor. Besides, the invention has a problem of providing a method offabricating such TFTs on the same substrate through the same steps.

Moreover, the invention has a problem of providing a wiring structure inwhich a reduction in wiring resistance is compatible with an increase inintegration in a semiconductor device including a large area integratedcircuit typified by an active matrix type liquid crystal display device,an EL display device, and an image sensor.

The present invention relates to a technique to solve such problems, andhas an object to realize a crystalline TFT capable of obtainingreliability comparable to or superior to a MOS transistor. Anotherobject of the invention is to increase reliability of a semiconductordevice including a large area integrated circuit in which variousfunctional circuits comprise such TFTs.

In order to solve the above problems, the present invention ischaracterized by making such a structure that an LDD region including aregion which overlaps with a gate electrode and a region which does notoverlap with the gate electrode is provided in one TFT.

Moreover, in a semiconductor device including a large area integratedcircuit typified by an active matrix type liquid crystal display device,an EL display device, and an image sensor, for the purpose of realizinga TFT having an optimum structure for every functional circuit, thepresent invention makes it possible to differentiate a ratio of a regionof an LDD region overlapping with a gate electrode to a region notoverlapping with the gate electrode for every TFT.

Moreover, in a semiconductor device including a large area integratedcircuit typified by an active matrix type liquid crystal display device,an EL display device, and an image sensor, for the purpose of realizinga gate wiring line effectively using Al or Cu of a low resistancematerial, such a wiring structure is made that a wiring line of a cladstructure is partially formed.

Thus, the structure of the present invention is such that in asemiconductor device including, over a substrate having an insulatingsurface, a semiconductor layer, a gate insulating film, a gateelectrode, and a gate wiring line connected to the gate electrode, thegate electrode comprises a first conductive layer or a first conductivelayer and a second conductive layer, and the gate wiring line isconstructed by a region comprising the same conductive layer as the gateelectrode and a region having a clad structure where a third conductivelayer is covered with the first conductive layer and the secondconductive layer.

The semiconductor layer includes a channel forming region, a firstimpurity region of one conductivity type, and a second impurity regionof the one conductivity type sandwiched between the channel formingregion and the first impurity region of the one conductivity type andbeing in contact with the channel forming region, and a part of thesecond impurity region of the one conductivity type overlaps with thegate electrode through the gate insulating film.

The first conductive layer and the second conductive layer applied tothe present invention use one kind or plural kinds of elements selectedfrom titanium (Ti), tantalum (Ta), tungsten (W), and molybdenum (Mo), ora compound containing the element as its main material. The thirdconductive layer comprises a low resistance conductive material typifiedby a kind of or plural kinds of elements selected from aluminum (Al) andcopper (Cu), or a compound containing the foregoing element as its mainmaterial.

The present invention can be applied to a semiconductor device includinga pixel region comprising an n-channel thin film transistor and a CMOScircuit comprising an n-channel thin film transistor and a p-channelthin film transistor.

However, in the foregoing CMOS circuit, it is not always necessary toapply the structure of the present invention to the p-channel TFT.

Moreover, another structure of the present invention is characterized bycomprising a step of forming a semiconductor layer over a substratehaving an insulating surface, a step of forming a gate insulating filmto be in contact with the semiconductor layer, a step of forming a firstconductive layer to be in contact with the gate insulating film, a stepof forming a second impurity region by selectively adding an impurityelement of one conductivity type to the semiconductor layer, a step offorming a third conductive layer to be in contact with the firstconductive layer, a step of forming a second conductive layer to be incontact with the first conductive layer and the third conductive layer,a step of forming a gate electrode from the first conductive layer andthe second conductive layer, a step of forming a gate wiring line fromthe first conductive layer, the second conductive layer, and the thirdconductive layer, a step of forming a first impurity region byselectively adding an impurity element of the one conductivity type tothe semiconductor layer, and a step of removing a part of the gateelectrode.

Moreover, the structure of the present invention is characterized bycomprising a step of forming a semiconductor layer over a substratehaving an insulating surface, a step of forming at least a firstisland-like semiconductor layer and a second island-like semiconductorlayer by removing a part of the semiconductor layer, a step of forming agate insulating film to be in contact with the first island-likesemiconductor layer and the second island-like semiconductor layer, astep of forming a first conductive layer to be in contact with the gateinsulating film, a step of forming a second impurity region by adding animpurity element of one conductivity type to at least a selected regionof the first island-like semiconductor layer, a step of forming a thirdconductive layer to be in contact with the first conductive layer, astep of forming a second conductive layer to be in contact with thefirst conductive layer and the third conductive layer, a step of forminga gate electrode from the first conductive layer and the secondconductive layer, a step of forming a gate wiring line from the firstconductive layer, the second conductive layer, and the third conductivelayer, a step of forming a first impurity region by adding an impurityelement of the one conductivity type to a selected region of the firstisland-like semiconductor layer, a step of forming a third impurityregion by adding an impurity element of a conductivity type opposite tothe one conductivity type to a selected region of the second island-likesemiconductor layer, and a step of removing a part of the gateelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E are sectional views showing fabricating steps of a TFT.

FIGS. 2A to 2C are sectional views showing fabricating steps of a TFtand a plan view of a CMOS circuit.

FIGS. 3A and 3B are sectional views showing fabricating steps of a TFT.

FIGS. 4A to 4C are sectional views showing fabricating steps of a TFT.

FIGS. 5A to 5F are sectional views showing fabricating steps of a TFT.

FIGS. 6A to 6C are sectional views showing fabricating steps of a TFTand a plan view of a CMOS circuit.

FIGS. 7A to 7F are sectional views showing fabricating steps of a TFT.

FIGS. 8A to 8C are sectional views showing fabricating steps of a TFTand a plan view showing a CMOS circuit.

FIGS. 9A to 9C are sectional views showing fabricating steps of a TFT.

FIGS. 10A to 10C are sectional views showing fabricating steps of a TFT.

FIG. 11 is a sectional view of an active matrix substrate.

FIG. 12 is a sectional view of an active matrix substrate.

FIGS. 13A and 13B are views showing fabricating steps of a liquidcrystal display device.

FIG. 14 is a sectional view of a liquid crystal display device.

FIG. 15 is a perspective view of an active matrix substrate.

FIG. 16 is a top view of a pixel region.

FIGS. 17A and 17B are a top view and a sectional view of a pixel region.

FIGS. 18A and 18B are views showing fabricating steps of a crystallinesilicon film.

FIGS. 19A and 19B are views showing fabricating steps of a crystallinesilicon film.

FIGS. 20A and 20B are views showing fabricating steps of a crystallinesilicon film.

FIGS. 21A and 21B are views showing fabricating steps of a crystallinesilicon film.

FIGS. 22A and 22B are views showing fabricating steps of a TFT.

FIG. 23 is a circuit block diagram of an active matrix type liquidcrystal display device according to a mode of the invention.

FIGS. 24A to 24D are views showing a structure of a TFT of theinvention.

FIGS. 25A to 25F are views showing examples of semiconductor devices.

FIGS. 26A to 26C are views for explaining the relation between a gateelectrode and an LDD region of the invention.

FIGS. 27A and 27B are views showing an outer appearance of an EL displaypanel and its sectional structure.

FIG. 28 is a view showing a sectional structure of a pixel portion of anEL display panel.

FIGS. 29A and 29B are views showing an upper structure of a pixelportion of an EL display panel and its circuit structure.

FIG. 30 is a view showing a sectional structure of an EL display panel.

FIGS. 31A to 31C are circuit diagrams of pixel portions of EL displaypanels.

FIG. 32 is a view showing an example of light transmittancecharacteristics of an antiferroelectric mixed liquid crystal.

FIGS. 33A to 33C and 34A to 34D are views showing examples ofsemiconductor devices.

DETAILED DESCRIPTION OF THE INVENTION

First, embodiment modes for carrying out the present invention will bedescribed below with reference to the drawings.

Embodiment Mode 1

An embodiment mode for carrying out the present invention will bedescribed with reference to FIGS. 1A to 1E and FIGS. 2A and 2C. Here, adescription will be made on an embodiment mode in which an n-channel TFTand a p-channel TFT are fabricated over the same substrate to form aninverter circuit as a basic structure of a CMOS circuit.

As a substrate 101, a glass substrate, a plastic substrate, a ceramicsubstrate, or the like may be used. Besides, a silicon substrate inwhich an insulating film such as a silicon oxide film or a siliconnitride film is formed on its surface, or a metal substrate typified bystainless may be used. Of course, a quartz substrate may be used.

An underlayer film 102 made of a silicon nitride film and an underlayerfilm 103 made of a silicon oxide film are formed on a main surface ofthe substrate 101 on which a TFT is to be formed. These underlayer filmsmay be formed by a plasma CVD method or a sputtering method, and areprovided in order to prevent a harmful impurity from diffusing from thesubstrate 101 to the TFT. For that purpose, it was appropriate that theunderlayer film 102 made of the silicon nitride film has a thickness of20 to 100 nm, typically 50 nm, and the underlayer film 103 made of thesilicon oxide film has a thickness of 50 to 500 nm, typically 150 to 200nm.

In addition to the above, in view of stress balance, two-layer structuremay be formed such that the underlayer film 102 comprises a firstsilicon nitride oxide film having a thickness of 10 to 100 nm andfabricated from SiH₄, NH₃, and N₂O by a plasma CVD method, and theunderlayer film 103 comprises a second silicon nitride oxide film havinga thickness of 100 to 200 nm and fabricated from SiH₄ and N₂O to belaminated.

Of course, although the underlayer film may be comprising only one ofthe underlayer film 102 made of the silicon nitride film and theunderlayer film 103 made of the silicon oxide film, it was mostpreferable to make the two-layer structure in view of reliability of theTFT.

As a semiconductor layer formed to be in contact with the underlayerfilm 103, it is desirable to use a crystalline semiconductor obtained byforming an amorphous semiconductor by such as a plasma CVD method, a lowpressure CVD method, or a sputtering method and crystallizing it by alaser annealing method or a thermal annealing method. It is alsopossible to apply a microcrystalline semiconductor formed by the filmformation method. As a semiconductor material which can be applied here,silicon (Si), germanium (Ge), silicon germanium alloy, and siliconcarbide can be enumerated, and in addition, a compound semiconductormaterial such as gallium arsenide may be used.

Alternatively, as a semiconductor layer formed over the substrate 101,an SOI (Silicon On Insulators) substrate in which a single crystalsilicon layer is formed may be used. Some kinds are known for the SOIsubstrate according to its structure and fabricating method, andtypically, SIMOX (Separation by Implanted Oxygen), ELTRAN (EpitaxialLayer Transfer: registered trademark by Canon Inc.), Smart-Cut(registered trademark by SOITEC Inc.) or the like may be used. Ofcourse, other SOI substrates may be used.

The semiconductor layer is formed to a thickness of 10 to 100 nm,typically 50 nm. Although hydrogen at a ratio of 10 to 40 atm % iscontained in an amorphous semiconductor film fabricated by a plasma CVDmethod, it is desirable that prior to a step of crystallization, a stepof heat treatment at 400 to 500° C. is carried out to remove hydrogenfrom the film so that the hydrogen content is made 5 atom % or less.Although an amorphous silicon film may be formed by another fabricatingmethod such as a sputtering method or an evaporation method, it isdesirable that the content of an impurity element contained in the film,such as oxygen or nitrogen, is sufficiently reduced.

Since the underlayer film and the amorphous semiconductor film can beformed by the same film formation method, it is appropriate that theunderlayer film 102, the underlayer film 103, and the semiconductorlayer are continuously formed. After the respective films are formed,the surfaces are not exposed to the air atmosphere, so that pollution ofthe surfaces can be prevented. As a result, it was possible to eliminateone of factors to cause fluctuation in characteristics of TFTs.

As a step of crystallizing the amorphous semiconductor film, a techniqueof a well-known laser annealing method or thermal annealing method maybe used. If a crystalline semiconductor film is formed by the techniqueof the thermal annealing method using a catalytic element, excellent TFTcharacteristics can be obtained.

A resist mask was formed by a well-known patterning method using a firstphotomask on the thus formed crystalline semiconductor film, andisland-like semiconductor layers 104 and 105 were formed by a dryetching method.

Next, a gate insulating film 106 containing silicon oxide or siliconnitride as its main material is formed on the surfaces of theisland-like semiconductor layers 104 and 105. The gate insulating film106 is formed by a plasma CVD method or a sputtering method, and it isappropriate that the thickness is made 10 to 200 nm, preferably 50 to150 nm.

A first conductive layer 107 and a third conductive layer 108 are formedon the surface of the gate insulating film 106. For the first conductivelayer 107, a conductive material containing an element selected from Ta,Ni, Mo and W as its main material is used. It is appropriate that thethickness of the first conductive layer 107 is made 5 to 50 nm,preferably 10 to 25 nm.

The thicknesses of the gate insulating film 106 and the first conductivelayer 107 are important. This is because in a doping step subsequentlycarried out, an impurity to give an n type is added to the semiconductorlayers 104 and 105 through the gate insulating film 106 and the firstconductive layer 107. Actually, in view of the thicknesses of the gateinsulating film 106 and the first conductive layer 107, the condition ofa doping step is determined. Here, if the thicknesses of the gateinsulating film 106 and the first conductive layer 107 are varied by 10%or more of a predetermined value, the concentration of an add impurityis reduced.

As the third conductive layer 108, a conductive material containing Alor Cu as its main material is used. For example, in the case where Al isused, an Al alloy in which an element selected from Ti, Si, and Sc isadded at a concentration of 0.1 to 5 atom % may be used. It isappropriate that the thickness of the third conductive layer is made 100to 1,000 nm, preferably 200 to 400 nm. This is formed as a wiringmaterial to reduce a wiring resistance of a gate wiring line or a gatebus line (FIG. 1A).

In the present invention, the gate wiring line is a wiring line whichcomprises the same material as the gate electrode on the gate insulatingfilm 106 and is connected to the gate electrode, and in the structure ofconnection to the gate electrode, the gate bus line is also regarded aspart of the gate wiring line.

Next, a second photomask was used to form a resist mask, and anunnecessary portion of the third conductive layer was removed, so thatpart of the gate bus line was formed (109 in FIG. 1B). In the case wherethe third conductive layer was made of Al, by a wet etching method usinga phosphoric acid solution, it was possible to remove the thirdconductive layer with good selectivity against the first conductivelayer as an under layer.

Then resist masks 110 and 111 covering channel forming regions of thesemiconductor layer 104 and the semiconductor layer 105 were formedthrough a third photomask. At this time, a resist mask 112 may be formedalso in a region where the wiring line is formed.

Then, a doping step for giving an n type was carried out. As an impurityelement to give the n type to a crystalline semiconductor material,phosphorus (P), arsenic (As), antimony (Sb) and the like are known.Here, phosphorus was used and the step was carried out by an ion dopingmethod using phosphine (PH₃). In this step, for the purpose of addingphosphorus through the gate insulating film 106 and the first conductivelayer 107 to the semiconductor layer thereunder, an acceleration voltagewas set as high as 80 keV. It is preferable that the concentration ofphosphorus added to the semiconductor layer is within the range of1×10¹⁶ to 1×10¹⁹ atoms/cm³, and here, it was made 1×10¹⁸ atoms/cm³.Then, regions 113 and 114 where phosphorus was added into thesemiconductor layer 105 were formed. A part of the region formed herewhere phosphorus was added is made a second impurity region functioningas an LDD region (FIG. 1B).

Thereafter, the resist masks 110, 111, and 112 were removed, and asecond conductive layer 115 was formed on the whole surface. The secondconductive layer 115 may be comprising the same material as the firstconductive layer 107, and a conductive material containing an elementselected from Ta, Ti, Mo, and W is used. It is appropriate that thethickness of the second conductive layer 115 is made 100 to 1,000 nm,preferably 200 to 500 nm (FIG. 1C).

Next, resist masks 116, 117, 118, and 119 were formed through a fourthphotomask. The fourth photomask is for forming a gate electrode, a gatewiring line, and a gate bus line of a p-channel TFT. Since a gateelectrode of an n-channel TFT is formed in a later step, the resist mask117 was formed such that a first conductive layer 122 and a secondconductive layer 123 remained on the semiconductor layer 105.

Unnecessary portions of the first conductive layer and the secondconductive layer were removed by a dry etching method. Then, gateelectrodes 120 and 121, gate wiring lines 124 and 125, and gate buslines 126 and 127 were formed.

The gate bus line is formed in such a clad structure that the thirdconductive layer 109 is covered with the first conductive layer 126 andthe second conductive layer 127. The third conductive layer comprises alow resistance material containing Al or Cu as its main material, and itwas possible to reduce wiring resistance.

A doping step was carried out to add an impurity element to give the ptype into a part of the semiconductor layer 104 where the p-channel TFTwas to be formed while the resist masks 116, 117, 118, and 119 were madeto remain as they were. As an impurity element to give the p type, boron(B), aluminum (Al), and gallium (Ga) are known. Here, boron was added asthe impurity element by an ion doping method using diborane (B₂H₆). Alsoin this step, an acceleration voltage was made 80 keV, and boron wasadded at a concentration of 2×10²⁰ atoms/cm³. As shown in FIG. 1D, thirdimpurity regions 152 and 153 where boron was added at a highconcentration were formed.

After the resist masks provided in FIG. 1D were removed, resist masks128, 129, and 130 were newly formed through a fifth photomask. The fifthphotomask is for forming a gate electrode of the n-channel TFT, and gateelectrodes 131 and 132 are formed by a dry etching method. At this time,the gate electrodes 131 and 132 are formed to overlap with part of thesecond impurity regions 113 and 114 (FIG. 1E).

After the resist masks 128, 129, and 130 were completely removed, resistmasks 133, 134, and 135 were formed. The resist mask 134 is formed insuch a shape as to cover the gate electrodes 131 and 132 of then-channel TFT and a part of the second impurity region. The resist mask134 determines an offset amount of the LDD region.

Then, a doping step of giving the n type was carried out. A firstimpurity region 137 which became a source region and a first impurityregion 136 which became a drain region were formed. Here, the step wascarried out by an ion doping method using phosphine (PH₃). Also in thisstep, for the purpose of adding phosphorus through the gate insulatingfilm 106 to the semiconductor layer thereunder, an acceleration voltagewas set as high as 80 keV. The concentration of phosphorus added in thisregion is high as compared with the prior doping step to give the ntype, and it is preferable that the concentration is made 1×10¹⁹ to1×10²¹ atoms/cm³, and here, it was made 1×10²⁰ atoms/cm³ (FIG. 2A).

Then, first interlayer insulating films 138 and 150 were formed on thesurfaces of the gate insulating film 106, the gate electrodes 120, 121,131, 132, the gate wiring lines 124 and 125, and the gate bus lines 126and 127. One of the first interlayer insulating films 150 was a siliconnitride film and was formed to a thickness of 50 nm. The other one ofthe first interlayer insulating films 138 was a silicon oxide film andwas formed to a thickness of 950 nm.

The one of the first interlayer insulating films 150 made of the siliconnitride film formed here was needed to carry out a subsequent heattreatment. This was effective to prevent oxidation of the surfaces ofthe gate electrodes 120, 121, 131, and 132, the gate wiring lines 124and 125, and the gate bus lines 126 and 127.

It was necessary to carry out the step of the heat treatment so that theimpurity element added at each concentration to give the n type or ptype was activated. This step may be carried out by a thermal annealingmethod using an electric heating furnace, the foregoing laser annealingmethod using an excimer laser, or a rapid thermal annealing method (RTA)using a halogen lamp. However, although the laser annealing method canmake activation at a low substrate heating temperature, it has beendifficult to make activation up to a region concealed under the gateelectrode. Thus, here, the step of activation was carried out by thethermal annealing method. The heat treatment was carried out in anitrogen atmosphere at 300 to 700° C., preferably 350 to 550° C., here450° C. for 2 hours.

Thereafter, after a predetermined resist mask was formed using a seventhphotomask, the first interlayer insulating films 138 and 150 were etchedto form contact holes reaching a source region and a drain region ofeach TFT. Then, source electrodes 139 and 140 and a drain electrode 141were formed. Although not shown, in this embodiment mode, the respectiveelectrodes were used as a three-layer electrode in which a Ti filmhaving a thickness of 100 nm, an Al film containing Ti and having athickness of 300 nm, and a Ti film having a thickness of 150 nm werecontinuously laminated by a sputtering method.

Through the foregoing steps, a channel forming region 145, firstimpurity regions 148 and 149, and second impurity regions 146 and 147were formed in the n-channel TFT of the CMOS circuit. Here, in thesecond impurity regions, regions (GOLD regions) 146 a and 147 aoverlapping with the gate electrode and regions (LDD regions) 146 b and147 b not overlapping with the gate electrode were formed, respectively.The first impurity region 148 functioned as a source region, and thefirst impurity region 149 functioned as a drain region.

On the other hand, in the p-channel, a channel forming region 142, andthird impurity regions 143 and 144 were formed. The third impurityregion 143 became a source region, and the third impurity region 144became a drain region (FIG. 2B).

FIG. 2C is a top view of an inverter circuit, and an A-A′ sectionalstructure of a TFT portion, a B-B′ sectional structure of a gate wiringline portion, and a C-C′ sectional structure of a gate bus line portioncorrespond to FIG. 2B. In the present invention, the gate electrode andthe gate wiring line comprise the first conductive layer and the secondconductive layer, and the gate bus line has a clad structure comprisingthe first conductive layer, the second conductive layer, and the thirdconductive layer.

Although FIGS. 1A to 1E and FIGS. 2A to 2C show the CMOS circuit formedby complementarily combining the n-channel TFT and the p-channel TFT asan example, the present invention can also be applied to an NMOS circuitusing an n-channel TFT or a pixel region of a display device.

Embodiment Mode 2

A structure of a TFT of the invention will be described in more detailwith reference to FIGS. 26A to 26C. Individual reference characters inFIGS. 26A and 26B are used to correspond to those of FIGS. 1A to 1E andFIGS. 2A to 2C. The second impurity region as the LDD region can bedivided into the second impurity region 146 a overlapping with the gateelectrodes 131 and 132 and the second impurity region 146 b notoverlapping with the gate electrodes. That is, the LDD region (Lov)overlapping with the gate electrode and the LDD region (Loff) notoverlapping with the gate electrode are formed.

The lengths of the regions Lov and Loff in the LDD region can be easilyrealized by pattering with three photomasks as shown in the embodimentmode 1. In the step shown in the embodiment mode 1, the LDD region isformed by the doping step in which the resist mask is formed with thethird photomask, and the overlap region (Lov) of the LDD is formed atthe same time as formation of the gate electrode using the fifthphotomask. Further, the LDD region (Loff) not overlapping is formed by aresist mask formed with a sixth photomask.

However, these three photomasks are also masks for patterning the gateelectrodes in addition to the object to form the resist masks, and thenumber of steps was not necessarily increased by making both thefunctions performed.

Thus, the freedom in design is given to the lengths of the regions Lovand Loff, and it was possible to set the lengths arbitrarily in view ofthe size of a TFT to be fabricated. This was a very effective method inthe case where TFTs with different driving voltages were fabricated forevery functional circuit in a large area integrated circuit. FIG. 26Cshows an example of design values of TFTs used for a logic circuitportion, a buffer circuit portion, an analog switch portion, and a pixelregion of an active matrix type liquid crystal display device. At thistime, in view of driving voltage of each TFT, it was possible tosuitably set, in addition to the channel length, the lengths of thesecond impurity region 146 a overlapping with the gate electrode and thesecond impurity region 146 b not overlapping with the gate electrode.

For example, in a TFT of a shift register circuit of a driver circuit ofa liquid crystal display device or a TFT of a buffer circuit, sinceimportance is basically attached to an ON characteristic, only theso-called GOLD structure may be adopted, and the second impurity region146 b not overlapping with the gate electrode was not necessarily neededto be provided. However, in the case of providing the region, it wassatisfactory if the value of the region Loff was set within the range of0.5 to 3 μm in view of the driving voltage. In view of the withstandvoltage, it was desirable that the value of the second impurity region146 b not overlapping with the gate electrode was made large as thedriving voltage became high.

In a TFT provided in a sampling circuit or a pixel region, for thepurpose of preventing an increase of an OFF current, in the case wherethe channel length was, for example, 3 μm, it was satisfactory if thelength of the second impurity region 146 a overlapping with the gateelectrode was made 1.5 μm and that of the second impurity region notoverlapping with the gate electrode was made 1.5 μm. Of course, thepresent invention is not limited to the design values indicated here,but the values may be suitably determined.

On the other hand, in the p-channel it was satisfactory if only thechannel forming region, the source region, and the drain region wereformed. Of course, the same structure as the n-channel TFT of thepresent invention may be adopted, since the PTFT has originally highreliability, it is preferable to secure an ON current and to take abalance to the n-channel TFT. In the case where the present invention isapplied to the CMOS circuit as shown in FIGS. 1A to 1E, it is especiallyimportant to take this characteristic balance. However, there is noproblem even if the structure of the present invention is applied to thep-channel TFT.

Embodiment Mode 3

First, in accordance with the same steps as those of the embodiment mode1, the state shown in FIG. 1E was obtained. As shown in FIG. 3A, afterthe resist masks 128, 129, and 130 were completely removed, resist masks301, 302, and 303 were formed. Although the resist mask 302 is formed insuch a shape as to cover a gate electrode of an n-channel TFT and a partof a second impurity region, and is for forming an LDD, it was designedhere that the resist mask was formed at only the drain side of then-channel TFT. Although the LDD region not overlapping with the gateelectrode prevents an increase of an OFF current, it was possible toobtain a sufficient effect even when the LDD was provided at only thedrain side (FIG. 3A).

The CMOS circuit shown in FIG. 3B was obtained by carrying outsubsequent steps similarly to the embodiment mode 1. A channel formingregion 145, first impurity regions 148 and 149, and a second impurityregion 147 were formed in the n-channel TFT. In the second impurityregion, a region (GOLD region) 147 a overlapping with the gate electrodeand a region (LDD region) 147 b not overlapping with the gate electrodewere formed. The first impurity region 148 became a source region andthe first impurity region 149 became a drain region.

Embodiment Mode 4

This embodiment mode of carrying out the present invention will bedescribed with reference to FIGS. 4A to 4C. First, in accordance withthe same steps as those in the embodiment mode 1, the state shown inFIG. 1C was obtained.

A photomask was used to form resist masks 401, 402, 403, and 404, andpart of the first conductive layer 107 and the second conductive layer108 were removed by a dry etching method. Thereafter, a doping step ofgiving an n type was carried out while the resist masks were used asthey were, so that regions 430, 431, 432, and 433 where phosphorus wasadded in the semiconductor layers 104 and 105 were formed.

Here, the resist masks were completely removed by using ashing and analkaline release solution. A photoresist mask was again formed and apatterning step by exposure from the rear side was carried out. At thistime, patterns of a gate electrode, a gate wiring line, and a gate busline played the same role as a photomask, so that resist masks 413, 414,415, and 416 were formed on the respective patterns. The exposure fromthe rear side is carried out using direct light and scattered light, andit was possible to form each of the resist masks on the gate electrodeand at its inside as shown in FIG. 4B by adjusting exposure conditionssuch as light intensity and an exposure time.

Then, part of the gate electrode, the gate wiring line, and the gate busline were removed by a dry etching method, so that gate electrodes 419,420, 421, and 422, gate wiring lines 423 and 424, and gate bus lines425, 426 and 427 were formed.

Then, resist masks 417 and 418 were formed and a doping step of givingthe p type was carried out using the gate electrodes 419 and 420 asmasks.

The CMOS circuit shown in FIG. 2B was formed by carrying out subsequentsteps similarly to the embodiment mode 1. A channel forming region 145,first impurity regions 148 and 149, and second impurity regions 146 and147 were formed in the n-channel TFT. Here, in the second impurityregions, regions (GOLD regions) 146 a and overlapping with the gateelectrode and regions (LDD regions) 146 b and 147 b not overlapping withthe gate electrode were formed. The first impurity region 148 functionedas a source region and the first impurity region 149 functioned as adrain region.

Embodiment Mode 5

Another embodiment mode for carrying out the present invention will bedescribed with reference to FIGS. 5A to 5F and FIGS. 6A to 6C. Here, adescription will be made on an embodiment mode in which an n-channel TFTand a p-channel TFT are fabricated on the same substrate so that aninverter circuit as a basic structure of a CMOS circuit is formed.

First, similarly to the embodiment mode 1, underlayer films 502 and 503were formed on a substrate 501, and further, island-like semiconductorlayers 504 and 505 made of crystalline semiconductor were formed.Further, a gate insulating film 506, a first conductive layer 507, and athird conductive layer 508 were formed, so that the state of FIG. 5A wasobtained.

Next, a resist mask was formed using a second photomask, and anunnecessary portion of the third conductive layer was removed, so that apart of a gate bus line was formed (510 in FIG. 5B). In the case wherethe third conductive layer comprises Al, it was possible to remove theportion with good selectivity against the first conductive layer as anunder layer by a wet etching method using a phosphoric acid solution.

Then, resist masks 511 and 512 covering channel forming regions of thesemiconductor layer 504 and the semiconductor layer 505 were formed witha third photomask. At this time, a resist mask 513 may be formed in aregion where a wiring line is formed.

A doping step of giving an n type was carried out by an ion dopingmethod using phosphine (PH₃). In this step, for the purpose of addingphosphorus through the gate insulating film 506 and the first conductivelayer 507 to the semiconductor layer thereunder, an acceleration voltagewas set as high as 80 keV. It is preferable that the concentration ofphosphorus added in the semiconductor layer is made 1×10¹⁶ to 1×10¹⁹atoms/cm³, and here, it was made 1×10¹⁸ atoms/cm³. Regions 514, 515,516, and 517 where phosphorus was added into the semiconductor layerwere formed. Part of the regions which were formed here and were addedwith phosphorus are made second impurity regions functioning as LDDregions (FIG. 5C).

A doping step of giving a p type to a part of the semiconductor layer504 where a p-channel TFT was to be formed was carried out using afourth photomask while the resist masks 518, 519, and 520 were made toremain as they were. Although boron (B), aluminum (Al), and gallium (Ga)are known as an impurity element to give the p type, boron was used hereas the impurity element and was added by an ion doping method usingdiborane (B₂H₆). Also in this step, an acceleration voltage was made 80keV, and boron was added at a concentration of 2×10²⁰ atoms/cm³. Asshown in FIG. 5D, third impurity regions 521 and 522 where boron wasadded at a high concentration were formed.

Thereafter, the resist masks 518, 519, and 520 were removed and a secondconductive layer 523 was formed on the whole surface. The secondconductive layer 523 may be comprising the same material as the firstconductive layer 507, and uses a conductive material containing anelement selected from Ta, Ti, Mo, and W as its main material. It isappropriate that the thickness of the second conductive layer 523 ismade 100 to 1,000 nm, preferably 200 to 500 nm (FIG. 5E).

Next, resist masks 524, 525, 526, and 527 were formed with a fifthphotomask. The first conductive layer and the second conductive layerwere subjected to a dry etching method so that unnecessary portions wereremoved. Then, gate electrodes 528, 529, 530 and 531, gate wiring lines532 and 533, and gate bus lines 534 and 535 were formed.

The gate bus line was formed in such a clad structure that the thirdconductive layer 510 was covered with the first conductive layer 534 andthe second conductive layer 535. The third conductive layer was made ofa low resistance material containing Al or Cu as its main material, andit was possible to reduce wiring resistance.

Then, resist masks 536, 537, and 538 were formed with a sixth photomask.The resist mask 537 was formed to cover the gate electrodes 530 and 531and a part of the second impurity region. The resist mask 537 was fordetermining the offset amount of the LDD region.

Then, a doping step of giving the n type was carried out. A firstimpurity region 540 which became a source region and a first impurityregion 541 which became a drain region were formed. Here, the step wascarried out by an ion doping method using phosphine (PH₃). Also in thisstep, for the purpose of adding phosphorus through the gate insulatingfilm 506 to the semiconductor layer thereunder, an acceleration voltagewas set as high as 80 keV. The concentration of phosphorus in thisregion is high as compared with the prior doping step of giving the ntype, and it is preferable that the concentration is made 1×10¹⁹ to1×10²¹ atoms/cm³, and here, it was made 1×10²⁰ atoms/cm³ (FIG. 6A).

Then, fifth interlayer insulating films 541 and 542 were formed on thesurfaces of the gate insulating film 506, the gate electrodes 528, 529,530, and 531, the gate wiring lines 532 and 533, and the gate bus lines534 and 535. The first interlayer insulating film 541 was a siliconnitride film and was formed to a thickness of 50 nm. The firstinterlayer insulating film 542 was made of a silicon oxide film and wasformed to a thickness of 950 nm.

It was necessary to carry out a step of heat treatment to activate theimpurity element added at each concentration to give the n type or ptype. This step may be carried out by a thermal annealing method usingan electric heating furnace, the foregoing laser annealing method usingan excimer laser, or a rapid thermal annealing method (RTA method) usinga halogen lamp. However, although the laser annealing method can makeactivation at a low substrate heating temperature, it has been difficultto make activation up to a region concealed under the gate electrode.Thus, here, the step of activation was carried out by the thermalannealing method. The heat treatment was carried out in a nitrogenatmosphere at 300 to 700° C., preferably 350 to 550° C., here 450° C.for 2 hours.

After a predetermined resist mask was formed, the first interlayerinsulating films 541 and 542 were etched to form contact holes reachinga source region and a drain region of each TFT. Then, source electrodes543 and 544 and a drain electrode 545 were formed. Although not shown,in this embodiment mode, this electrode was used as an electrode ofthree-layer structure in which a Ti film having a thickness of 100 nm,an Al film containing Ti and having a thickness of 300 nm, and a Ti filmhaving a thickness of 150 nm were continuously formed by a sputteringmethod.

Through the foregoing steps, a channel forming region 549, firstimpurity regions 552 and 553, and second impurity regions 550 and 551were formed in the n-channel TFT of the CMOS circuit. Here, in thesecond impurity regions, regions (GOLD regions) 550 a and 551 aoverlapping with the gate electrode and regions (LDD regions) 550 b and551 b not overlapping with the gate electrode were formed, respectively.The first impurity region 552 became a source region and the firstimpurity region 553 became a drain region.

On the other hand, in the p-channel TFT, a channel forming region 546and third impurity regions 547 and 548 were formed. The third impurityregion 547 became a source region and the third impurity region 548became a drain region (FIG. 5B).

FIG. 5C is a top view of an inverter circuit, and an A-A′ sectionalstructure of a TFT portion, a B-B′ sectional structure of a gate wiringline portion, and a C-C′ sectional structure of a gate bus line portioncorrespond to FIG. 5B. In the present invention, the gate electrode andthe gate wiring line comprise the first conductive layer and the secondconductive layer, and the gate bus line has a clad structure comprisingthe first conductive layer, the second conductive layer, and the thirdconductive layer.

Although FIGS. 5A to 5F and FIGS. 6A to 6C show the CMOS circuit formedby complementarily combining the n-channel TFT and the p-channel TFT asan example, the present invention can also be applied to an NMOS circuitusing an n-channel TFT or a pixel region of a liquid crystal displaydevice.

Embodiment Mode 6

Another embodiment mode for carrying out the present invention will bedescribed with reference to FIGS. 7A to 7F and FIGS. 8A to 8C. Here, adescription will be made on an embodiment mode in which an n-channel TFTand a p-channel TFT are fabricated on the same substrate and an invertercircuit as a basic structure of a CMOS circuit is formed.

First, similarly to the embodiment mode 5, underlayer films 702 and 703were formed on a substrate 701, and island-like semiconductor layers 704and 705 made of crystalline semiconductor were formed. Further, a gateinsulating film 706, a first conductive layer 707, and a thirdconductive layer 708 were formed to obtain the state of FIG. 7A.

Next, a resist mask was formed, and an unnecessary portion of the thirdconductive layer was removed, so that a part of a gate bus line wasformed (710 in FIG. 7B). In the case where the third conductive layerwas made of Al, it was possible to remove the portion with goodselectivity against the first conductive layer as an under layer by awet etching method using a phosphoric acid solution.

Then, resist masks 711 and 712 covering channel forming regions of thesemiconductor layer 704 and the semiconductor layer 705 were formed. Atthis time, a resist mask 713 may be formed in a region where a wiringline is formed.

Then, a doping step of giving the n type was carried out by an iondoping method using phosphine (PH₃). In this step, for the purpose ofadding phosphorus through the gate insulating film 706 and the firstconductive layer 707 to the semiconductor layer thereunder, anacceleration voltage was set as high as 80 keV. It is preferable thatthe concentration of phosphorus added into the semiconductor layer ismade 1×10¹⁶ to 1×10¹⁹ atoms/cm³, and here, it was made 1×10¹⁸ atoms/cm³.Then, regions 714, 715, 716, and 717 where phosphorus was added into thesemiconductor layer were formed. Part of the regions which were formedhere and were added with phosphorus are made second impurity regionsfunctioning as LDD regions (FIG. 7C).

Then, resist masks 718, 719 and 720 were formed and prior to a dopingstep to give a p type, a portion where the first conductive layer wasexposed was removed by an etching method. Then a doping step of givingthe p type was carried out. Since the first conductive layer was removedhere, it was possible to lower the acceleration voltage in the iondoping method. Boron was used as the impurity element and was added bythe ion doping method using diborane (B₂H₆). The acceleration voltagewas made 40 keV, and boron was added at a concentration of 2×10²°atoms/cm³. As shown in FIG. 7D, third impurity regions 724 and 725 whereboron was added at a high concentration were formed.

The subsequent steps may be carried out in accordance with theembodiment mode 5, and as shown in FIG. 8A, resist masks 739, 740, and741 were formed, and first impurity regions 742 and 743 were formed by adoping step to give the n type. Then, a channel forming region 752,first impurity regions 755 and 756, and second impurity regions 753 and754 were formed in the n-channel TFT of the CMOS circuit. Here, in thesecond impurity regions, regions (GOLD regions) 753 a and 754 aoverlapping with gate electrode and regions (LDD regions) 753 b and 754b not overlapping with the gate electrode were formed, respectively. Thefirst impurity region 755 became a source region and the first impurityregion 756 became a drain region.

On the other hand, in the p-channel TFT, a channel forming region 749and third impurity regions 750 and 751 were formed. The third impurityregion 750 became a source region and the third impurity region 751became a drain region (FIG. 8B).

FIG. 8C is a top view of an inverter circuit, and an A-A′ sectionalstructure of a TFT portion, a B-B′ sectional structure of a gate wiringline portion, and a C-C′ sectional structure of a gate bus line portioncorrespond to FIG. 8B. In the present invention, the gate electrode andthe gate wiring line comprise the first conductive layer and the secondconductive layer, and the gate bus line has a clad structure comprisingthe first conductive layer, the second conductive layer, and the thirdconductive layer.

Although FIGS. 7A to 7F and FIGS. 8A to 8C show the CMOS circuit formedby complementarily combining the n-channel TFT and the p-channel TFT asan example, the present invention can also be applied to an NMOS circuitusing an n-channel TFT or a pixel region of a liquid crystal displaydevice.

Next, embodiments of the present invention will be described below withreference to the drawings.

Embodiment 1

In this embodiment, a structure of the present invention will bedescribed with reference to FIGS. 9A to 11 and a description will bemade on a method of simultaneously fabricating a pixel region and a CMOScircuit as a basic structure of a driver circuit provided on itsperiphery.

In FIGS. 9A to 9C, as a substrate 901, an alkali-free glass substratetypified by, for example, a substrate of 1737 glass made by Corning Inc.was used. An underlayer film 902 was formed on the surface of thesubstrate 901 on which TFTs were to be formed. As the underlayer film902, although not shown, a silicon nitride film having a thickness of 25to 100 nm, here, 50 nm, and a silicon oxide film having a thickness of50 to 300 nm, here, 150 nm were formed. The underlayer film 902 may beformed using only a silicon nitride film or a silicon nitride oxidefilm.

The underlayer film 902 may be comprising such a two-layer structurethat a first silicon nitride oxide film having a thickness of 10 to 100nm is fabricated by a plasma CVD method from SiH₄, NH₃, and N₂O, and asecond silicon nitride oxide film fabricated from SiH₄ and N₂O andhaving a thickness of 100 to 200 nm is laminated thereon.

The first silicon nitride oxide film is formed by using a parallel platetype plasma CVD method. The first silicon nitride oxide film was formedunder the conditions that SiH₄ of 10 SCCM, NH₃ of 100 SCCM, and N₂O of20 SCCM were introduced into a reaction chamber, substrate temperaturewas made 325° C., reaction pressure was made 40 Pa, discharge powerdensity was made 0.41 W/cm², and discharge frequency was made 60 MHz. Onthe other hand, the second silicon nitride oxide film was formed underthe conditions that SiH₄ of 4 SCCM and N₂O of 400 SCCM were introducedinto the reaction chamber, substrate temperature was made 400° C.,reaction pressure was made 40 Pa, discharge power density was made 0.41W/cm², and discharge frequency was made 60 MHz. These films can also becontinuously formed only by changing the substrate temperature andchanging the reaction gas. The first silicon nitride oxide film isformed so that the inner stress becomes tensile stress when thesubstrate is considered as the center. Although the second siliconnitride oxide film is made to have inner stress in the same direction,it is appropriate that the absolute value of the stress is made smallerthan that of the first silicon nitride oxide film.

Next, an amorphous silicon film having a thickness of 50 nm was formedon the underlayer film 902 by a plasma CVD method. It is desirable thataccording to the hydrogen content, the amorphous silicon film ispreferably heated at 400 to 550° C. for several hours to carry out adehydrogenating process so that the hydrogen content is made 5 atom % orless, and a step of crystallization is carried out. Although theamorphous silicon film may be formed by another fabricating method suchas a sputtering method or an evaporation method, it is desirable thatimpurity elements such as oxygen and nitrogen contained in the film aresufficiently decreased in advance.

Here, both the underlayer film and the amorphous silicon film arefabricated by the plasma CVD method, and at this time, the underlayerfilm and the amorphous silicon film may be continuously formed invacuum. By making such a step that the underlayer film was not exposedto the air after it was formed, it became possible to prevent pollutionof the surface and it was possible to reduce fluctuation incharacteristics of TFTs fabricated.

As a step of crystallizing the amorphous silicon film, a well-knownlaser annealing method or a thermal annealing method may be used. Inthis embodiment, the laser annealing method was used, and a pulseoscillation type KrF excimer laser light was linearly condensed and wasirradiated to the amorphous silicon film to form a crystalline siliconfilm.

In this embodiment, as the semiconductor layer, although the crystallinesilicon film is formed from the amorphous silicon film, a microcrystalsilicon film may be used, or a crystalline silicon film may be directlyformed.

The thus formed crystalline silicon film was patterned by using a firstphotomask to form island-like semiconductor layers 903, 904 and 905.

Next, a gate insulating film 906 containing silicon oxide or siliconnitride as its main material was formed to cover the island-likesemiconductor layers 903, 904, and 905. As the gate insulating film 906,a silicon nitride oxide film having a thickness of 10 to 200 nm,preferably 50 to 150 nm may be formed by a plasma CVD method using N₂Oand SiH₄ as a raw material. Here, the film was formed to a thickness of100 nm.

Then, a first conductive film 907 and a third conductive film 908 wereformed on the surface of the gate insulating film 906. The firstconductive film 907 may be comprising a semiconductor film containing anelement selected from Ta, Ti and W or containing these elements as itsmain material. It is necessary that the thickness of the firstconductive film 907 is made 5 to 50 nm, preferably 10 to 30 nm. Here, aTa film having a thickness of 20 nm was formed.

In the case where the Ta film is used, the film can be formed by asputtering method. For the Ta film, Ar is used as a sputtering gas. If asuitable amount of Xe or Kr is added into the sputtering gas, it ispossible to relieve inner stress of a formed film and to prevent peelingof the film. Although the resistivity of the Ta film with a phase á isabout 20 μÙcm and can be used for a gate electrode, the resistivity ofthe Ta film with a phase â is about 180 μÙcm and is unsuitable for agate electrode. However, since a TaN film has a crystal structure closeto the phase á, if the Ta film is formed thereon, the Ta film with thephase á can be easily obtained. Thus, although not shown, the TaN filmhaving a thickness of 10 to 50 nm may be formed under the firstconductive film. Similarly, although not shown, it is effective to forma silicon film having a thickness of about 2 to 20 nm and doped withphosphorus (P) under the first conductive layer. By this, improvement inadhesion of the conductive film formed thereon and prevention ofoxidation can be realized, and it is possible to prevent a very smallamount of alkali metal element contained in the first conductive film orthe second conductive film from diffusing to the gate insulating film906. In all events, it is preferable that the resistivity of the firstconductive film is made a value within the range of 10 to 50 μÙcm.

In addition, a W film can also be used, and in that case, argon (Ar) gasand nitrogen (N₂) gas are introduced to form the W film having athickness of 200 nm. It is also possible to form the W film by a thermalCVD method using tungsten hexafluoride (WF₆). In all events, it isnecessary to decrease the resistance in order to use the film as a gateelectrode, and it is desirable that the resistivity of the W film ismade 20 μÙcm or less. Although the resistivity of the W film can bedecreased by enlarging crystal grains, in the case where a lot ofimpurity elements such as oxygen are contained in the W film,crystallization is obstructed and the resistance is increased. Fromthis, in the case of the sputtering method, a W target with a purity of99.9999% is used, and further, the W film is formed while carefulattention is paid to prevent mixture of an impurity from a vapor phaseat the film formation, so that a resistivity of 9 to 20 μÙcm can berealized.

A conductive material containing Al or Cu as its main material is usedfor the third conductive layer 908. For example, in the case where Al isused, an Al alloy added with an element selected from Ti, Si, and Sc at0.1 to 5 atom % may be used. It is appropriate that the third conductivelayer is formed to a thickness of 100 to 1,000 nm, preferably 200 to 400nm. This is formed as a wiring material to decrease wiring resistance ofa gate wiring line or a gate bus line.

Similarly, the third conductive layer is useful in forming a wiring lineconnecting an input terminal provided at an end portion of the substrate901 to each circuit, and the wiring resistance can be decreased.

An impurity element to give the n type or p type conductivity may beadded to the semiconductor film used as the first conductive film. Amethod of fabricating this semiconductor film may be carried out inaccordance with a well-known method. For example, the film can befabricated by a low pressure CVD method under the condition that thesubstrate temperature is made 450 to 500° C., and disilane (Si₂H₆) of250 SCCM and helium (He) of 300 SCCM are introduced. At the same time,an n-type semiconductor film may be formed by mixing PH₃ of 0.1 to 2%into Si₂H₆ (FIG. 9A).

Boron (B) of a concentration of 1×10¹⁶ to 5×10¹⁸ atoms/cm³ may bepreviously added in the island-like semiconductor layers. The boron (B)is added to control a threshold voltage, and other elements may besubstituted as long as the same effect can be obtained.

Next, a resist mask was formed using a second photomask, and anunnecessary portion of the third conductive layer was removed to form agate bus line and part of other wiring lines (909 and 910 in FIG. 9B).In the case where the third conductive layer was made of Al, it waspossible to remove the portion with good selectivity against the firstconductive layer as an under layer by a wet etching method using aphosphoric acid solution.

Then, resist masks 911, 912, 915 and 916 were formed to cover thesemiconductor layer 903 and channel forming regions of the semiconductorlayers 904 and 905. At this time, resist masks 913 and 914 may be formedalso in the region where wiring lines are formed.

Then, a doping step of giving the n type was carried out. Phosphorus(P), arsenic (As), antimony (Sb), etc. are known as an impurity elementto give the n type to a crystalline semiconductor material. Here,phosphorus was used and an ion doping method using phosphine (PH₃) wascarried out. In this step, for the purpose of adding phosphorus throughthe gate insulating film 906 and the first conductive layer 907 to thesemiconductor layer under the films, an acceleration voltage was set ashigh as 80 KeV. It is preferable that a concentration of phosphorusadded in this region is made a value within the range of 1×10¹⁶ to 1×10′atoms/cm³, and here, it was made 1×10¹⁸ atoms/cm³. Then, regions 917,918, 919, 920, and 921 where phosphorus (P) was added in thesemiconductor layer were formed. Here, part of the formed regions wherephosphorus was added are made second impurity regions functioning as LDDregions (FIG. 9B).

Thereafter, the resist masks were removed and a second conductive layer922 was formed on the whole surface. The second conductive layer 922 maybe comprising the same material as the first conductive layer 907, and aconductive material containing an element selected from Ta, Ti, Mo, andW is used. It is appropriate that the second conductive layer 922 isformed to a thickness of 100 to 1,000 nm, preferably 200 to 500 nm (FIG.9C).

Next, resist masks 923, 924, 925, 926, 927, and 928 were formed with afourth photomask. The fourth photomask is for forming a gate electrodeof a p-channel TFT, gate wiring lines of a CMOS circuit and a pixelregion, gate bus lines, and other wiring lines. Since a gate electrodeof an n-channel TFT was formed in a later step, the resist masks 924 and928 were formed so that the first conductive layers 931 and 942 and thesecond conductive layers 932 and 941 remained on the semiconductorlayer.

Unnecessary portions of the first conductive layer and the secondconductive layer were removed by a dry etching method. Then, gateelectrodes 929 and 930, gate wiring line 933 and 934, and gate bus lines935, 937, 938, and 940 were formed.

The respective gate bus lines were formed as a clad structure in whichthe third conductive layer 936 and 939 was covered with the firstconductive layer 935 and 938 and the second conductive layer 937 and940. The third conductive layer was made of a low resistance materialcontaining Al or CU as its main material, and it was possible to reducethe wiring resistance.

A doping step of giving the p type to a part of the semiconductor layer903 where the p-channel TFT was to be formed was carried out while theresist masks 923, 924, 925, 926, 927, and 928 were made to remain asthey were. Although boron (B), aluminum (Al) and gallium (Ga) are knownas an impurity element to give the p type, boron was used here as theimpurity element and was added by an ion doping method using diborane(B₂H₆). Also in this case, an acceleration voltage was made 80 keV, andboron was added at a concentration of 2×10²⁰ atoms/cm³. As shown in FIG.10A, third impurity regions 943 and 944 where boron was added at a highconcentration were formed.

After the resist masks provided in FIG. 10A were removed, resist masks945, 946, 947, 948, 949, 950, and 951 were newly formed with a fifthphotomask. The fifth photomask is for forming gate electrodes ofn-channel TFTs, and gate electrodes 952, 953, 954, 955, 956, and 957were formed by a dry etching method. At this time, the gate electrodes952, 953, 954, 955, 956 and 957 were formed to overlap with part of thesecond impurity regions 917, 918, 919, 920, and 921 (FIG. 10B).

After the resist masks were completely removed, new resist masks 960,961, 962, 963, 964 and 965 were formed. The resist masks 961, 964, 965were formed to cover the gate electrodes 952, 953, 954, 955, 956 and 957of the n-channel TFTs and part of the second impurity regions. Theresist masks 961, 964, 965 determine the offset amounts of LDD regions.

The resist masks 960, 961, 962, 963, 964 and 965 were formed using asixth photomask, and a doping step of giving the n type was carried out.First impurity regions 967 and 968 which became source regions and firstimpurity regions 966, 969, and 970 which became drain regions wereformed. Here, the step was carried out by an ion doping method usingphosphine (PH₃). Also in this step, for the purpose of adding phosphorusthrough the gate insulating film 906 to the semiconductor layer underthe film, an acceleration voltage was set as high as 80 KeV. Aconcentration of phosphorus in the regions is high as compared with theprior doping step of giving the n type, and it is preferable that theconcentration is made 1×10¹⁹ to 1×10²¹ atoms/cm³, and here, it was made1×10²⁰ atoms/cm³ (FIG. 10C).

After the steps up to FIG. 10C were completed, a step of forming firstinterlayer insulating films 971 and 972 was carried out. First, asilicon nitride film 971 was formed to a thickness of 50 nm. The siliconnitride film 971 was formed by a plasma CVD method under the conditionthat SiH₄ of 5 SCCM, NH₃ of 40 SCCM, and N₂ of 100 SCCM were introduced,the pressure was made 0.7 Torr, and a high frequency power of 300 W wasapplied. Subsequently, as the first interlayer insulating film 972, asilicon oxide film having a thickness of 950 nm was formed under thecondition that tetraethyl orthosilicate (TEOS) of 500 SCCM and O₂ of 50SCCM were introduced, the pressure was made 1 Torr, and a high frequencypower of 200 W was applied.

Then, a step of heat treatment was carried out. It was necessary tocarry out the step of heat treatment to activate the impurity elementadded at each concentration to give the n type or p type. This step maybe carried out by a thermal annealing method using an electric heatingfurnace, the foregoing laser annealing method using an excimer laser, ora rapid thermal annealing method (RTA method) using a halogen lamp.Here, the step of activation was carried out by the thermal annealingmethod. The heat treatment was carried out in a nitrogen atmosphere at300 to 700° C., preferably 350 to 550° C., here, 450° C. for 2 hours.

The first interlayer insulating films 971 and 972 were patterned to formcontact holes reaching a source region and a drain region of each TFT.Then, source electrodes 973, 974, and 975 and drain electrodes 976 and977 were formed. Although not shown, in this embodiment, the respectiveelectrodes were formed as a three-layer electrode in which a Ti filmhaving a thickness of 100 nm, an Al film containing Ti and having athickness of 300 nm, and a Ti film having a thickness of 150 nm werecontinuously formed by a sputtering method.

Through the foregoing steps, a channel forming region 981, firstimpurity regions 984 and 985, and second impurity regions 982 and 983were formed in the n-channel TFT of the CMOS circuit. Here, in thesecond impurity regions, regions (GOLD regions) 982 a and 983 aoverlapping with gate electrode and regions (LDD regions) 982 b and 983b not overlapping with the gate electrode were formed, respectively. Thefirst impurity region 984 became a source region and the first impurityregion 985 became a drain region.

In the p-channel TFT, the gate electrode of a clad structure wassimilarly formed, and a channel forming region 978, and third impurityregions 979 and 980 were formed. The third impurity region 979 became asource region and the third impurity region 980 became a drain region.

The pixel TFT has a multi gate structure, and channel forming regions986 and 991, first impurity regions 989, 990 and 994, and secondimpurity regions 987, 988, 992 and 993 were formed. Here, in the secondimpurity regions, regions 987 a, 988 a, 992 a and 993 a overlapping withthe gate electrode, and regions 987 b, 988 b, 992 b and 993 b notoverlapping with the gate electrode were formed.

In this way, as shown in FIG. 11, an active matrix substrate in whichthe CMOS circuit and the pixel portion were formed over the substrate901 was fabricated. At the same time, a holding capacitance portion wasformed at the drain side of the n-channel TFT of the pixel portion.

Embodiment 2

This embodiment will be described with reference to FIG. 12. In FIG. 12,a light shielding film 996 was formed to correspond to a region where asemiconductor layer of a pixel portion was formed, and was providedbefore an underlayer film 997 was formed. It is appropriate that thelight shielding film 996 comprises a metal film of Ti, Ta, Cr, W or thelike. It may also be comprising tungsten silicide (WSi). Morepreferably, it may be comprising a two-layer structure of a WSi film anda Si film. It is appropriate that the light shielding film 996 is formedto a thickness of 100 to 400 nm, typically 200 nm.

Steps of forming TFTs on the light shielding film 996 may be carried outin accordance with the embodiment 1. Then, an active matrix substrateshown in FIG. 12 is formed. It is preferable that such an active matrixsubstrate is used for a liquid crystal display device for projection.The light shielding film cut off scattered light, so that it waspossible to effectively prevent an increase in an OFF current of ann-channel TFT of the pixel portion.

Embodiment 3

In this embodiment, a description will be made on an example where acrystalline semiconductor film used as the semiconductor layer in theembodiment 1 is formed by a thermal annealing method using a catalyticelement. In the case where the catalytic element is used, it isdesirable to use a technique disclosed in Japanese Patent ApplicationLaid-open No. Hei. 7-130652 or No. Hei. 8-78329.

Here, an example of a case where the technique disclosed in JapanesePatent Application Laid-open No. Hei. 7-130652 is applied to the presentinvention will be described in FIGS. 18A and 18B. First, a silicon oxidefilm 1802 was formed on a silicon substrate 1801 and an amorphoussilicon film 1803 was formed thereon. Further, a nickel acetate saltsolution containing nickel of 10 ppm in terms of weight was applied toform a nickel containing layer 1804 (FIG. 18A).

Next, after a dehydrogenating step at 500° C. for 1 hour was carriedout, a heat treatment at 500 to 650° C. for 4 to 12 hours, for example,at 550° C. for 8 hours was carried out, so that a crystalline siliconfilm 1805 was formed. The crystalline silicon film 1805 obtained in thisway had extremely superior crystallinity (FIG. 18B).

The technique disclosed in Japanese Patent Application Laid-open No.Hei. 8-78329 is such that selective crystallization of an amorphoussemiconductor film is made possible by selectively adding a catalyticelement. A case where the technique is applied to the present inventionwill be described with reference to FIGS. 19A and 19B.

First, a silicon oxide film 1902 was formed on a glass substrate 1901,and an amorphous silicon film 1903 and a silicon oxide film 1904 werecontinuously formed thereon. At this time, the thickness of the siliconoxide film 1904 was made 150 nm.

Next, the silicon oxide film 1904 was patterned to selectively formopening portions 1905. Thereafter, a nickel acetate salt solutioncontaining nickel of 10 ppm in terms of weight was applied. By this, anickel containing layer 1906 was formed, and the nickel containing layer1906 was brought into contact with the amorphous silicon film 1902 atonly the bottoms of the opening portions 1905 (FIG. 19A).

Next, a heat treatment at 500 to 650° C. for 4 to 24 hours, for example,at 570° C. for 14 hours was carried out, so that a crystalline siliconfilm 1907 was formed. In this crystallizing process, a portion withwhich nickel is in contact is first crystallized, and crystal growthprogresses in the lateral direction therefrom. The thus formedcrystalline silicon film 1907 comprises a collective of rod-like orneedle-like crystals, and each crystal macroscopically grows withcertain directionality. Thus, there is an advantage that crystallinityis uniform (FIG. 19B).

In the foregoing two techniques, instead of nickel (Ni), an element suchas germanium (Ge), iron (Fe), palladium (Pd), tin (Sn), lead (Pb),cobalt (Co), platinum (Pt), copper (Cu), or gold (Au) may be used.

If a crystalline semiconductor film (including a crystalline siliconfilm, a crystalline silicon germanium film, etc.) is formed by using thetechnique as described above, and patterning is carried out, asemiconductor layer of a crystalline TFT can be formed. Althoughsuperior characteristics can be obtained in the TFT fabricated from thecrystalline semiconductor by using the technique of this embodiment,high reliability has been required because of that. However, when theTFT structure of the present invention is adopted, it becomes possibleto fabricate a TFT which utilizes the technique of this embodiment tothe utmost.

Embodiment 4

In this embodiment, a description will be made on an example in which asa method of forming a semiconductor layer used in the embodiment 1,after a crystalline semiconductor film is formed using an amorphoussemiconductor film as an initial film and using a catalytic element, astep of removing the catalytic element from the crystallinesemiconductor film is carried out. As a method thereof, this embodimentuses a technique disclosed in Japanese Patent Application Laid-open No.10-247735, No. Hei. 10-135468, or No. Hei. 10-135469.

The technique disclosed in the publications is such that a catalyticelement used for crystallization of an amorphous semiconductor film isremoved after crystallization by using a gettering function ofphosphorus. By using the technique, it is possible to reduce aconcentration of a catalytic element in a crystalline semiconductor filmto 1×10¹⁷ atoms/cm³ or less, preferably 1×10¹⁶ atoms/cm³ or less.

A structure of this embodiment will be described with reference to FIGS.20A and 20B. Here, an alkali-free glass substrate typified by asubstrate of 1737 glass made by Corning Inc. was used. FIG. 20A shows astate in which an underlayer film 2002 and a crystalline silicon film2003 were formed by using the technique disclosed in the embodiment 3.Then, a silicon oxide film 2004 for masking was formed to a thickness of150 nm on the surface of the crystalline silicon film 2003, and openingportions were provided by patterning, so that regions where thecrystalline silicon film was exposed were provided. Then, a step ofadding phosphorus was carried out so that a region 2005 added withphosphorus was provided in the crystalline silicon film.

In this state, when a heat treatment at 550 to 800° C. for 5 to 24hours, for example, at 600° C. for 12 hours was carried out in anitrogen atmosphere, the region 2005 where phosphorus was added in thecrystalline silicon film functioned as a gettering site, so that it waspossible to segregate the catalytic element remaining in the crystallinesilicon film 2003 into the region 2005 added with phosphorus.

By removing the silicon oxide film 2004 for masking and the region 2005added with phosphorus, it was possible to obtain a crystalline siliconfilm in which the concentration of the catalytic element used in thestep of crystallization was reduced to 1×10¹⁷ atoms/cm³ or less. It waspossible to use this crystalline silicon film without any change as thesemiconductor layer of the TFT of the present invention described in theembodiment 1.

Embodiment 5

In this embodiment, a description will be made on another example inwhich a semiconductor layer and a gate insulating film are formed in astep of fabricating a TFT of the present invention described in theembodiment 1. A structure of this embodiment will be described withreference to FIGS. 21A and 21B.

Here, a substrate having heat resistance of at least about 700 to 1100°C. was necessary and a quartz substrate 2101 was used. The techniquedisclosed in the embodiment 3 and the embodiment 4 was used to form acrystalline semiconductor. For the purpose of making this asemiconductor layer of a TFT, this was patterned into island-likeregions so that semiconductor layers 2102 and 2103 were formed. A gateinsulating film 2104 covering the semiconductor layers 2102 and 2103 wascomprising a film containing silicon oxide as its main material. In thisembodiment, a silicon nitride oxide film having a thickness of 70 nm wasformed by a plasma CVD method (FIG. 21A).

Then, a heat treatment was carried out in an atmosphere containing ahalogen (typically, chlorine) and oxygen. In this embodiment, the heattreatment was carried out at 950° C. for 30 minutes. Incidentally, itwas appropriate that the processing temperature was selected within therange of 700 to 1100° C. and the treatment time was selected within therange of 10 minutes to 8 hours (FIG. 21B).

As a result, under the condition of this embodiment, thermal oxidationfilms were formed between the semiconductor layers 2102, 2103 and thegate insulating film 2104, so that gate insulating films 2107 wereformed. Moreover, in the process of oxidation in the halogen atmosphere,an impurity contained in the gate insulating film 2104 and thesemiconductor layers 2102 and 2103, especially a metal impurity elementwas combined with the halogen to form a compound, so that it waspossible to remove the impurity element into the vapor phase.

The gate insulating films 2107 fabricated through the above steps had ahigh withstand voltage, and interfaces between the semiconductor layers2105, 2106 and the gate insulating films 2107 were very excellent. Inorder to obtain the structure of the TFT of the present invention, itwas satisfactory if the subsequent steps were carried out in accordancewith the embodiment 1.

Embodiment 6

In this embodiment, a description will be made on an example in which ina method of fabricating an active matrix substrate through stepsdescribed in the embodiment 1 after forming a crystalline semiconductorfilm by a method described in the embodiment 3, a catalytic element usedin a step of crystallization is removed by gettering. In the embodiment1, the semiconductor layers 903, 904 and 905 shown in FIG. 9A werecrystalline silicon films fabricated by using a catalytic element. Atthis time, since the catalytic element used in the step ofcrystallization remained in the semiconductor layers, it was desirableto carry out a step of gettering.

Here, the process up to the step shown in FIG. 10B was carried out as itwas. Then, the resist masks 945, 946, 947, 948, 949, 950 and 951 wereremoved.

As shown in FIG. 22A, new resist masks 2201, 961, 962, 963, 964 and 965were formed. Then, a doping step of giving the n type was carried out.Regions 2202, 2203, 966, 967, 968, 969, and 970 added with phosphoruswere formed in the semiconductor layer.

Although boron of an impurity element to give the p type was alreadyadded in the regions 2202 and 2203 where phosphorus was added, since theconcentration of phosphorus is 1×10¹⁹ to 1×10²¹ atoms/cm³ and phosphoruswas added at a concentration of about one half of a concentration ofboron, it did not have any influence ON characteristics of the p-channelTFT.

In this state, a heat treatment at 400 to 800° C. for 1 to 24 hours, forexample, at 600° C. for 12 hours was carried out in a nitrogenatmosphere. By this step, it was possible to activate the added impurityelements to give the n type and p type. Further, the regions added withphosphorus became gettering sites, so that it was possible to segregatethe catalytic element remaining after the crystallization step. As aresult, it was possible to remove the catalytic element from channelforming regions (FIG. 22B).

After the step of FIG. 22B was completed, by carrying out subsequentsteps in accordance with the steps of the embodiment 1 to form the stateof FIG. 11, it was possible to fabricate an active matrix substrate.

Embodiment 7

In this embodiment, a description will be made on a process offabricating an active matrix type liquid crystal display device from anactive matrix substrate fabricated with the techniques of theembodiments 1 to 6.

A passivation film 1301 was formed to the active matrix substrate in thestate of FIG. 11. The passivation film 1301 was made of a siliconnitride film having a thickness of 50 nm. Further, a second interlayerinsulating film 1302 made of organic resin was formed to a thickness ofabout 1,000 nm as shown in FIG. 13A. As the organic resin, polyimide,acryl, polyimidoamide, etc. may be used. As advantages obtained by usingthe organic resin film, it is possible to enumerate such points that afilm formation method is simple, parasitic capacitance can be reducedsince its relative dielectric constant is low, and flatness is superior.An organic resin film other than the above may be used. Here, polyimideof such a type that thermal polymerization was made after application tothe substrate was used, and was fired at 300° C. to form the film.

Further, a third interlayer insulating film was formed as shown in FIG.13B. The third interlayer insulating film 1304 was formed by using anorganic resin film of polyimide or the like. A contact hole reaching thedrain electrode 977 was formed in the third interlayer insulating film1304, the second interlayer insulating film 1302, and the passivationfilm 1301, and a pixel electrode 1305 was formed. With respect to thepixel electrode 1305, it is appropriate that a transparent conductivefilm is used in the case where a transmission type liquid crystaldisplay device is formed, and a metal film is used in the case where areflection type liquid crystal display device is formed. Here, for thepurpose of making the transmission type liquid crystal display device,an indium-tin oxide (ITO) film having a thickness of 100 nm was formedby a sputtering method, so that the pixel electrode 1305 was formed.

Next, as shown in FIG. 14, an orientated film 1401 was formed on thethird interlayer insulating film 1304 and the pixel electrode 1305. Ingeneral, a polyimide resin is often used for an oriented film of aliquid crystal display device. A transparent electrode 1403 and anoriented film 1404 were formed on an opposite side substrate 1402. Theoriented film was subjected to a rubbing process after formation so thatliquid crystal molecules were made to be oriented in parallel and with acertain pretilt angle.

After the foregoing steps, the active matrix substrate on which thepixel region and the CMOS circuit were formed and the opposite substratewere bonded to each other by a well-known cell assembling step through asealing material, a spacer (both are not shown), and the like.Thereafter, a liquid crystal material 1405 was injected between both thesubstrates, and complete sealing was made by a sealing agent (notshown). Thus, the active matrix type liquid crystal display device shownin FIG. 14 was completed.

Next, a structure of an active matrix type liquid crystal display deviceof this embodiment will be described with reference to FIGS. 15 and 16.FIG. 15 is a perspective view of an active matrix substrate of thisembodiment. The active matrix substrate is constructed by a pixelportion 1501, a scanning (gate) line driver circuit 1502, and a data(source) line driver circuit 1503 formed on a glass substrate 901. Apixel TFT 1500 is an n-channel TFT, and the driver circuits provided atthe periphery comprise a CMOS circuit as a base. The scanning (gate)line driver circuit 1502 and the data (source) line driver circuit 1503are connected to the pixel portion 1501 through a gate wiring line 1603and a source wiring line 1604, respectively.

FIG. 16 is a top view of the pixel portion 1501 and is a top view ofabout one pixel. The pixel TFT is an n-channel TFT. A gate electrode1602 formed to be connected with the gate wiring line 1603 intersectsthrough a not-shown gate insulating film with a semiconductor layer 1601thereunder. Although not shown, a source region, a drain region, and afirst impurity region are formed in the semiconductor layer. At a drainside of the pixel TFT, a holding capacitance 1607 comprises thesemiconductor layer, the gate insulating film, and an electrode made ofthe same material as the gate electrode. A sectional structure alongline A-A′ and line B-B′ shown in FIG. 16 corresponds to the sectionalview of the pixel region shown in FIG. 14.

In this embodiment, although the pixel TFT 1500 has a double gatestructure, a single gate structure may be adopted, or a multi gatestructure of a triple gate may be adopted. The structure of the activematrix substrate of the invention is not limited to the structure ofthis embodiment. Since the structure of the present invention ischaracterized in the structure of a gate electrode, and the structure ofa source region, a drain region and other impurity regions of asemiconductor layer provided through a gate insulating film, otherstructures may be suitably determined by an operator.

Embodiment 8

In this embodiment, another structural example of a pixel region of anactive matrix type liquid crystal display device will be described withreference to FIGS. 17A and 17B.

FIG. 17A is a top view of a pixel region and is a top view of about onepixel. In the pixel region, an n-channel TFT is provided. An A-A′section of FIG. 17A corresponds to FIG. 17B. A gate wiring line has aclad type structure and comprises a first conductive layer 1714, asecond conductive layer 1716, and a third conductive layer 1715. Gateelectrodes connected to the gate wiring line comprise first conductivelayers 1717 and 1719 and second conductive layers 1718 and 1720.Although not shown in FIG. 17A, a source region, a drain region and afirst impurity region are formed in a semiconductor layer. A substrate1701 has an underlayer film comprising SiN 1702 and an underlayer filmcomprising SiO₂ 1703 thereon. A semiconductor layer in the pixel regionhas first impurity regions 1704 (source region) and 1712 (drain region),second impurity regions 1705, 1707, 1709, 1711 (LDD regions), channelforming regions 1706, 1710, a low concentration impurity region 1713. Ata drain side of the pixel TFT, a holding capacitance comprises thesemiconductor layer, a gate insulating film, and an electrode comprisingthe same material as the gate electrode. Holding capacitance electrodes1721 and 1722 are formed over a gate insulating film. Source electrode1723 and drain electrode 1724 are connected to the semiconductor layer.Then, a first interlayer insulating film 1730, a passivation film 1725,a second interlayer insulating film 1726, third insulating film 1727,and a pixel electrode 1728 are formed.

Embodiment 9

FIG. 23 shows an example of a circuit structure of an active matrix typeliquid crystal display device shown in the embodiment 7. The activematrix type liquid crystal display device of this embodiment includes asource signal line side driver circuit 2301, a gate signal line sidedriver circuit (A) 2307, a gate signal line side driver circuit (B)2311, a precharge circuit 2312 and a pixel region 2306.

The source signal line side driver circuit 2301 includes a shiftregister circuit 2302, a level shifter circuit 2303, a buffer circuit2304 and a sampling circuit 2305.

The gate signal line side driver circuit (A) 2307 includes a shiftregister circuit 2308, a level shifter circuit 2309, and a buffercircuit 2310. The gate signal line side driver circuit (B) 2311 has alsothe same structure.

Here, an example of driving voltage of each circuit will be shown. Theshift register circuit 2302 and 2308 had 10 to 16 V, and the levelshifter circuits 2303 and 2309, the buffer circuits 2304 and 2310, thesampling circuit 2305, and the pixel region 2306 had 14 to 16 V. Withrespect to the sampling circuit 2305 and the pixel region 2306, thevalue was an amplitude of an applied voltage, and generally reversedvoltages were alternately applied.

In the present invention, it is easy to make the lengths of secondimpurity regions, which become LDD regions, different from each other onthe same substrate in view of driving voltages of n-channel TFTs, and itwas possible to form the optimum shapes for TFTs constituting therespective circuits through the same step.

FIG. 24A shows a structural example of a TFT of a shift registercircuit. An n-channel TFT of the shift register circuit has a singlegate, and a second impurity region which becomes an LDD region isprovided at only a drain side. Here, the lengths of an LDD region (GOLDregion) 206 a overlapping with a gate electrode and an LDD region 206 bnot overlapping with the gate electrode may be determined in accordancewith FIG. 26C, and the regions can be formed such that the region 206 ahas a length of 2.0 μm and the region 206 b has a length of 1.0 μm.

FIG. 24B shows a structural example of a TFT of a level shifter circuitor a buffer circuit. An n-channel TFT of these circuits is made to havea double gate, and a second impurity region which becomes an LDD regionis provided at a drain side. For example, the length of each of LDDregions (GOLD regions) 205 a and 205 c overlapping with gate electrodescan be made 2.5 μm, and the length of each of LDD regions 205 b and 205d not overlapping with the gate electrodes can be made 2.5 μm. Ofcourse, the gate electrode of the TFT is not limited to the double gatestructure. It may be a single gate structure or a multi gate structurehaving a plurality of gate electrode.

Since a level shifter circuit and a buffer circuit need to have a highcurrent driving performance, TFT's in the level shifter circuit and thebuffer circuit are easy to deteriorate due to kink effect. However, thedeterioration of the TFT's can be prevented by forming the LDD region asshown in FIG. 24B.

FIG. 24C shows a structural example of a TFT of a sampling circuit.Although an n-channel TFT of this circuit has a single gate, since thepolarity is inverted, a second impurity region which becomes an LDDregion is provided at both sides of a source side and a drain side. Itis preferable that the lengths of LDD regions (GOLD regions) 205 a and206 a overlapping with a gate electrode and the lengths of LDD regions205 b and 206 b not overlapping with the gate electrode are respectivelymade equal to each other. For example, the lengths of the LDD regions(GOLD regions) overlapping with the gate electrode can be made 1.5 μm,and the lengths of the LDD regions 205 b and 206 b not overlapping withthe gate electrode can be made 1.0 μm.

FIG. 24D shows a structural example of a pixel region. Although ann-channel TFT of this circuit has a multi gate, since the polarity isinverted, a second impurity region which becomes an LDD region isprovided at both sides of a source side and a drain side. For example,the lengths of LDD regions (GOLD regions) 205 a, 205 b, 206 a, and 206 coverlapping with a gate electrode can be made 1.5 μm, and the lengths ofLDD regions 206 b and 206 d not overlapping with the gate electrode canbe made 1.5 μm.

Embodiment 10

In this embodiment, a description will be given on a semiconductordevice incorporating an active matrix liquid crystal display device madefrom a TFT circuit of the present invention, with reference to FIGS. 25Ato 25F, FIGS. 33A to 33C and FIGS. 34A to 34D.

As such a semiconductor device, a portable information terminal (anelectronic book, a mobile computer or a cellular phone), a video camera,a still-image camera, a personal computer, TV etc. may be enumerated.Examples of those are shown in FIGS. 25A to 25F.

FIG. 25A is a cellular phone that is composed of a main body 9001, asound output section 9002, a sound input section 9003, a display device9004, operation switches 9005, and an antenna 9006. The presentinvention can be applied to the sound output section 9002, the soundinput section 9003 and the display device 9004 having an active matrixsubstrate.

FIG. 25B shows a video camera that is comprised of a main body 9101, adisplay device 9102, a voice input unit 9103, operation switches 9104, abattery 9105, and an image receiving unit 9106. The present invention isapplicable to the voice input unit 9103, the display device 9102 havingan active matrix substrate and the image receiving unit 9106.

FIG. 25C shows a mobile computer that is comprised of a main body 9201,a camera unit 9202, an image receiving unit 9203, operation switches9204, and a display device 9205. The present invention can be applied tothe image receiving unit 9203 and the display device 9205 having anactive matrix substrate.

FIG. 25D shows a head mount display that is comprised of a main body9301, a display device 9302 and arm portions 9303. The present inventioncan be applied to the display device 9302. Further, although not shown,the present invention can also be used for other signal controlcircuits.

FIG. 25E shows a rear-type projector that is comprised of a main body9401, a light source 9402, display device 9403, a polarization beamsplitter 9404, reflectors 9405 and 9406, and a screen 9407. The presentinvention can be applied to the display device 9403.

FIG. 25F shows a portable electronic book that is comprised of a mainbody 9501, display devices 9502, 9503, a memory medium 9504, anoperation switch 9505 and an antenna 9506. The book is used to displaydata stored in a mini-disk or a DVD, or a data received with theantenna. The display devices 9502, 9503 are direct-vision type displaydevices, to which the present invention may be applied.

FIG. 33A shows a personal computer comprising a main body 9601, an imageinputting unit 9602, a display device 9603 and a key board 9604.

FIG. 33B shows a player that employs a recording medium in whichprograms are recorded (hereinafter referred to as recording medium), andcomprises a main body 9701, a display device 9702, a speaker unit 9703,a recording medium 9704, and an operation switch 9705. Incidentally,this player uses as the recording medium a DVD (Digital Versatile Disc),CD and the like to appreciate music and films, play games, and connectto the Internet.

FIG. 33C shows a digital camera comprising a main body 9801, a displaydevice 9802, an eye piece section 9803, operation switches 9804, and animage receiving unit (not shown).

FIG. 34A shows a front-type projector comprising a projection device3601 and a screen 3602. The present invention is applicable to theprojection device 3601 and other signal control circuits.

FIG. 34B shows a rear-type projector comprising a main body 3701, aprojection device 3702, a mirror 3703, and a screen 3704. The presentinvention is applicable to the projection device 3702 (specially, iteffects in case of 50-100 inch projector) and other signal controlcircuits.

FIG. 34C is a diagram showing an example of the structure of theprojection devices 3601, 3702 in FIGS. 34A and 34B. The projectiondevice 3601 or 3702 comprises a light source optical system 3801,mirrors 3802, 3804 to 3806, dichroic mirrors 3803, a prism 3807, liquidcrystal display devices 3808, phase difference plates 3809, and aprojection optical system 3810. The projection optical system 3810 iscomposed of an optical system including a projection lens. This exampleshows an example of “Three plate type” but not particularly limitedthereto. For instance, the invention may be applied also to a “Singleplate type” optical system. Further, in the light path indicated by anarrow in FIG. 34C, an optical system such as an optical lens, a filmhaving a polarization function, a film for adjusting a phase difference,and an IR film may be suitably provided by a person who carries out theinvention.

FIG. 34D is a diagram showing an example of the structure of the lightsource optical system 3801 in FIG. 34C. In this embodiment, the lightsource optical system 3801 comprises a reflector 3811, a light source3812, lens arrays 3813, 3814, a polarization conversion element 3815,and a condenser lens 3816. The light source optical system shown in FIG.34D is merely an example, and is not particularly limited to theillustrated structure. For example, a person who carries out theinvention is allowed to suitably add to the light source optical systeman optical system such as an optical lens, a film having a polarizationfunction, a film for adjusting a phase difference, and an IR film.

Other than those, the present invention may be applied to an imagesensor and an EL display element. The present invention thus has so wideapplication range that it is applicable to electronic equipment in anyfield.

Embodiment 11

In this embodiment, a description will be made on an example in which anEL (electroluminescence) display panel (also called an EL displaydevice) is fabricated using the present invention.

FIG. 27A is a top view of an EL display panel using the presentinvention. In FIG. 27A, reference numeral 10 designates a substrate; 11,a pixel portion; 12, a source side driver circuit; and 13, a gate sidedriver circuit. The respective driver circuits lead to an FPC 17 throughwiring lines 14 to 16 and are connected to an external equipment.

At this time, a sealing material (also called a housing material) 18 isprovided so as to surround at least the pixel portion, preferably thedriver circuits and the pixel portion. As the sealing material 18, aglass plate or plastic plate having a recess portion capable ofsurrounding a component portion may be used, or an ultraviolet raycuring resin may be used. In the case where the plastic plate having therecess portion capable of surrounding the component portion is used asthe sealing material 18, the plate is bonded to the substrate 10 by anadhesive 19, and a sealed space is formed against the substrate 10. Atthis time, an EL element is put in such a state that it is completelyenclosed in the sealed space, and it is completely insulated from theouter air.

Further, it is preferable that an inert gas (argon, helium, nitrogen,etc.) is filled in a gap 20 between the sealing material 18 and thesubstrate 10, or a drying agent such as barium oxide is provided in thegap. By this, it is possible to suppress deterioration of the EL elementdue to moisture or the like.

FIG. 27B is a view showing a sectional structure of the EL display panelof this embodiment. A TFT 22 for a driver circuit (here, a CMOS circuitmade of a combination of an n-channel TFT and a p-channel TFT is shown)and a TFT 23 for a pixel portion (here, only a TFT for controlling acurrent to the EL element is shown) are formed on an underlayer film 21of the substrate 10. As the TFT 22 for the driver circuit, the n-channelTFT or p-channel TFT shown in FIG. 2 may be used. As the TFT 23 for thepixel portion, the n-channel TFT or p-channel TFT shown in FIG. 2 may beused.

After the TFT 22 for the driver circuit and the TFT 23 for the pixelportion are completed by using the present invention, a pixel electrode27 electrically connected to a drain of the TFT 23 for the pixel portionand made of a transparent conductive film is formed on an interlayerinsulating film (flattening film) 26 made of a resin material. As thetransparent conductive film, a compound (called ITO) of indium oxide andtin oxide or a compound of indium oxide and zinc oxide may be used.After the pixel electrode 27 is formed, an insulating film 28 is formed,and an opening portion is formed over the pixel electrode 27.

Next, an EL layer 29 is formed. The EL layer 29 may be made of alaminate structure by freely combining well-known EL materials (a holeinjection layer, a hole transport layer, a light emitting layer, anelectron transport layer or an electron injection layer) or asingle-layer structure. A well-known technique may be used to determinewhat structure is formed. The EL material includes a low molecularmaterial and a high molecular (polymer) material. In the case where thelow molecular material is used, an evaporation method is used. In thecase where the high molecular material is used, it is possible to use asimple method such as a spin coating method, a printing method or an inkjet method.

In this embodiment, the EL layer is formed by using a shadow mask and bythe evaporation method. A luminescent layer (a red luminescent layer, agreen luminescent layer, and a blue luminescent layer) capable ofemitting light with different wavelengths is formed for every pixel byusing the shadow mask, so that color display becomes possible. Inaddition to that, there are a system in which a color conversion layer(CCM) and a color filter are combined and a system in which a whiteluminescent layer and a color filter are combined, and any of themethods may be used. Of course, an EL display device of monochromaticluminescence may be made.

After the EL layer 29 is formed, a cathode 30 is formed thereon. It isdesirable that moisture and oxygen existing at the interface between thecathode 30 and the EL layer 29 is removed to the utmost degree. Thus,such contrivance is necessary that the EL layer 29 and the cathode 30are continuously formed in vacuum, or the EL layer 29 is formed in aninert gas atmosphere and the cathode 30 is formed without opening to theair. In this embodiment, a multi-chamber system (cluster tool system)film forming apparatus is used so that the foregoing film formation ismade possible.

In this embodiment, a laminate structure of a LiF (lithium fluoride)film and an Al (aluminum) film is used for the cathode 30. Specifically,a LiF (lithium fluoride) film having a thickness of 1 nm is formed onthe EL layer 29 by an evaporation method, and an aluminum film having athickness of 300 nm is formed thereon. Of course, a MgAg electrode of awell-known cathode material may be used. The cathode 30 is connected tothe wiring line 16 in a region designated by 31. The wiring line 16 is apower source supply line for supplying a predetermined voltage to thecathode 30, and is connected to the FPC 17 through a conductive pastematerial 32.

For the purpose of electrically connecting the cathode 30 with thewiring line 16 in the region 31, it is necessary to form a contact holein the interlayer insulating film 26 and the insulating film 28. Thismay be formed at the time of etching of the interlayer insulating film26 (at the time of formation of the contact hole for the pixelelectrode) and at the time of etching of the insulating film 28 (at thetime of formation of the opening portion prior to the formation of theEL layer). Besides, when the insulating film 28 is etched, theinterlayer insulating film 26 may also be etched at the same time. Inthis case, if the interlayer insulating film 26 and the insulating film28 comprise the same resin material, it is possible to make the shape ofthe contact hole excellent.

The wiring line 16 is electrically connected to the FPC 17 through thegap (it is filled with the adhesive 19) between the sealing material 18and the substrate 10. Although the description has been made on thewiring line 16, the other wiring lines 14 and 15 are also electricallyconnected to the FPC 17 through the portion under the sealing material18 in the same manner.

In the EL display panel having the structure as described above, thepresent invention can be used. Here, FIG. 28 shows a more detailedsectional structure of the pixel portion, FIG. 29A shows its upperstructure, and FIG. 29B shows a circuit diagram. Since common referencecharacters are used in FIG. 28, FIG. 29A and FIG. 29B, they may bereferred to one another.

In FIG. 28, a switching TFT 2402 provided over a substrate 2401 isformed by using an n-channel TFT of the present invention (for example,a TFT of the example 1 shown in FIG. 2). In this embodiment, although adouble gate structure is adopted, since a great difference does notexist in a structure and a fabricating process, the explanation isomitted. However, by adopting the double gate structure, such astructure is obtained that two TFTs are substantially connected inseries. Thus, there is a merit that an OFF current value can be reduced.Although the double gate structure is adopted in this embodiment, asingle gate structure may be adopted, or a triple gate structure or amulti gate structure having more gates may be adopted. Alternatively, itis also possible to form the TFT by using a p-channel TFT of the presentinvention.

A current controlling TFT 2403 is formed by using an n-channel TFT ofthe present invention. At this time, a drain wiring line 35 of theswitching TFT 2402 is electrically connected to a gate electrode 37 ofthe current controlling TFT through a wiring line 36. A wiring linedesignated by 38 is a gate wiring line for electrically connecting gateelectrodes 39 a and 39 b of the switching TFT 2402.

At this time, it has a very important meaning that the currentcontrolling TFT 2403 has the structure of the present invention. Sincethe current controlling TFT is a device for controlling the amount ofcurrent flowing through the EL element, it is also such a device that alot of current flows and there is a high fear of deterioration by heator deterioration by hot carriers. Thus, it is very effective to use thestructure of the present invention that an LDD region is provided at adrain side of the current controlling TFT through a gate insulating filmso as to overlap with a gate electrode (strictly speaking, a side wallfunctioning as the gate electrode).

In this embodiment, although the current controlling TFT 2403 is shownas a single gate structure, a multi gate structure in which a pluralityof TFTs are connected in series may be adopted. Besides, such astructure may be adopted that a plurality of TFTs are connected inparallel so that a channel forming region is substantially divided intoplural regions and radiation of heat can be carried out at highefficiency. Such a structure is effective as a countermeasure againstdeterioration due to heat.

As shown in FIG. 29A, a wiring line which becomes the gate electrode 37of the current controlling TFT 2403 overlaps with a drain wiring line 40of the current controlling TFT 2403 through an insulating film at aregion designated by 2404. At this time, a capacitor is formed at theregion designated by 2404. This capacitor 2404 functions as a capacitorfor holding voltage applied to the gate of the current controlling TFT2403. Incidentally, the drain wiring line 40 is connected to a currentsupply line (power source line) 2501 and is always applied with aconstant voltage.

A first passivation film 41 is provided on the switching TFT 2402 andthe current controlling TFT 2403, and a flattening film 42 made of aresin insulating film is formed thereon. It is very important that astep due to a TFT is flattened by using the flattening film 42. Since asubsequently formed EL layer is very thin, there is a case where poorluminescence occurs due to the step. Thus, it is desirable to makeflattening prior to formation of a pixel electrode so that the EL layercan be formed on a surface with the utmost flatness.

Reference numeral 43 designates a pixel electrode (cathode of the ELelement) made of a conductive film having high reflectivity, and iselectrically connected to the drain of the current controlling TFT 2403.As the pixel electrode 43, it is preferable to use a low resistanceconductive film such as an aluminum alloy film, a copper alloy film, ora silver alloy film, or a laminate film of those. Of course, a laminatestructure using other conductive films may be used.

A light emitting layer 44 is formed in a groove (corresponding to apixel) comprising banks 44 a and 44 b made of insulating films(preferably, resin). Although only one pixel is shown here, lightemitting layers corresponding to colors of R (Red), G (Green) and B(Blue) may be separately formed. As an organic EL material which is madea light emitting layer, 6 conjugated polymer material is used. Astypical polymer materials, polyparaphenylene vinylene (PPV),polyvinylcarbazole (PVK), polyfluorene, and the like can be enumerated.

Although various types exist for the PPV organic EL material, forexample, a material disclosed in “H. Schenk, H. Becker, O. Gelsen, E.Kluge, W. Kreuder, and H. Spreitzer, “Polymers for Light EmittingDiodes,” Euro Display, Proceedings, 1999, p. 33-37,” or Japanese PatentApplication Laid-open No. Hei. 10-92576 may be used.

As a specific light emitting layer, it is appropriate thatcyanopolyphenylene vinylene is used for a light emitting layer emittingred light, polyphenylene vinylene is used for a light emitting layeremitting green light, and polyphenylene vinylene or polyalkylenephenylene is used for a light emitting layer emitting blue light. It isappropriate that the film thickness is made 30 to 150 nm (preferably 40to 100 nm).

However, the above embodiments are only examples of organic resinmaterials capable of being used for the light emitting layer, and it isnot necessary to limit the invention to those. The EL layer (layer inwhich light emission is made and carrier movement for that is made) maybe formed by freely combining a light emitting layer, a charge transportlayer, and a charge injection layer.

For example, although this embodiment shows an example in which apolymer material is used for the light emitting layer, a low molecularorganic EL material may be used. It is also possible to use an inorganicresin material such as silicon carbide for the charge transport layer orthe charge injection layer.

In this embodiment, the EL layer is made to have such a laminatestructure that a hole injection layer 46 made of PEDOT (polythiophene)or PAni (polyaniline) is provided on a light emitting layer 45. An anode47 made of a transparent conductive layer is provided on the holeinjection layer 46. In the case of this embodiment, since light producedin the light emitting layer 45 is emitted toward the upper surface side(toward a portion above the TFT), the anode must be translucent. As thetransparent conductive film, although a compound of indium oxide and tinoxide or a compound of indium oxide and zinc oxide may be used, since itis formed after the light emitting layer and the hole injection layerhaving low heat resistance are formed, it is preferable that thetransparent conductive film can be formed at the lowest possibletemperature.

At the point when the anode 47 has been formed, an EL element 2405 iscompleted. The EL element here indicates the pixel electrode (cathode)43, the light emitting layer 45, the hole injection layer 46 and thecapacitor formed at the anode 47. As shown in FIG. 29A, since the pixelelectrode 43 roughly coincides with the area of a pixel, the whole pixelfunctions as the EL element. Thus, a usage efficiency of light emissionbecomes very high, and bright image display becomes possible.

In this embodiment, a second passivation film 48 is provided on theanode 47. As the second passivation film 48, a silicon nitride film or asilicon nitride oxide film is preferable. This object is to isolate theEL element from the outside, which has both of a meaning to preventdeterioration due to oxidation of the organic resin material and ameaning to prevent degassing from the organic resin material. By this,the reliability of the EL display device can be raised.

As described above, the EL display panel of the present inventionincludes the pixel portion made of a pixel having the structure as shownin FIG. 28, and includes a switching TFT which has a sufficiently lowOFF current value and a current controlling TFT which is strong againsthot carrier injection. Thus, the EL display panel having highreliability and enabling excellent image display can be obtained.

The structure of this embodiment can be freely combined with thestructure of the embodiment modes 1 to 6 and the embodiments 1 to 6.Besides, it is effective to use the EL display panel of this embodimentas a display portion of an electronic equipment of the embodiment 10.

Embodiment 12

In this embodiment, a description will be made on a structure in whichthe structure of the EL element 2405 is inverted in the pixel portionshown in the embodiment 11. FIG. 30 is used for the description.Incidentally, since different points from the structure of FIG. 29A areonly a portion of an EL element and a current controlling TFT, thedescription of other portions is omitted.

In FIG. 30, a current controlling TFT 2601 is formed by using ap-channel TFT of the present invention. The embodiment 1 may be referredto for fabricating steps.

In this embodiment, a transparent conductive film is used as a pixelelectrode (anode) 50. Specifically, a conductive film made of a compoundof indium oxide and zinc oxide is used. Of course, a conductive filmmade of a compound of indium oxide and tin oxide may be used.

After banks 51 a and 51 b made of an insulating film are formed, a lightemitting layer 52 made of polyvinylcarbazole is formed by application ofa solution. An electron injection layer 53 made of potassiumacetylacetonate (expressed by acacK) and a cathode 54 made of aluminumalloy are formed thereon. In this case, the cathode 54 functions also asa passivation film. In this way, an EL element 2602 is formed.

In the case of this embodiment, light generated in the light emittinglayer 53 is emitted as indicated by an arrow toward the substrate onwhich TFTs are formed. In the case where the structure of thisembodiment is adopted, it is preferable that the current controlling TFT2601 comprises a p-channel TFT.

Incidentally, the structure of this embodiment can be freely combinedwith the structure of the embodiment modes 1 to 6 and the embodiments 1to 6. Also, it is effective to use the EL display panel of thisembodiment as a display portion of an electronic equipment of theembodiment 10.

Embodiment 13

In this embodiment, an example of a pixel having a structure differentfrom the circuit diagram shown in FIG. 29B will be described withreference to FIGS. 31A to 31C. In this embodiment, 2701 designate asource wiring line of a switching TFT 2702; 2703, a gate wiring line ofthe switching TFT 2702; 2704, a current controlling TFT; 2705, acapacitor; 2706, 2708, current supply lines; and 2707, an EL element.

FIG. 31A shows an example of a case where a current supply line 2706 iscommon to two pixels. That is, this example is characterized in that twopixels are formed axisymmetrically with respect to the current supplyline 2706. In this case, since the number of power supply lines can bereduced, the pixel portion can be further made fine.

FIG. 31B shows an example of a case where a current supply line 2708 isprovided in parallel with a gate wiring line 2703. Although FIG. 31Bshows a structure in which the current supply line 2708 and the gatewiring line 2703 are provided so that they do not overlap with eachother, if both are wiring lines formed in different layers, it is alsopossible to provide them so that they overlap with each other through aninsulating film. In this case, since an occupied area can be made commonto the power supply line 2708 and the gate wiring line 2703, the pixelportion can be made further fine.

FIG. 31C shows an example characterized in that a current supply line2708 is provided in parallel with gate wiring lines 2703 similarly tothe structure of FIG. 31B, and further, two pixels are formed to becomeaxisymmetric with respect to the current supply line 2708. It is alsoeffective to provide the current supply line 2708 in such a manner thatit overlaps with either one of the gate wiring lines 2703. In this case,since the number of power supply lines can be reduced, the pixel portioncan be further fine.

The structure of this embodiment can be freely combined with thestructure of the embodiment 11 or 12. It is effective to use an ELdisplay panel having a pixel structure of this embodiment as a displayportion of an electronic equipment of the embodiment 10.

Embodiment 14

Although the embodiment 11 shown in FIGS. 29A and 29B has such astructure that the capacitor 2404 for holding a voltage applied to thegate of the current controlling TFT 2403 is provided, the capacitor 2404may be omitted.

In the case of the embodiment 13, since an n-channel TFT of the presentinvention as shown in FIG. 28 is used for the current controlling TFT2403, it includes an LDD region provided to overlap with a gateelectrode through a gate insulating film. Although a parasiticcapacitance generally called gate capacitance is formed in thisoverlapping region, this embodiment is characterized in that thisparasitic capacitance is positively used as a substitution of thecapacitor 2404.

Since the capacitance of this parasitic capacitance is changed by anarea where the gate electrode overlaps with the LDD region, it isdetermined by the length of the LDD region contained in the overlappingregion.

Also in the structures of FIGS. 31A, 31B and 31C, the capacitor 2705 canbe omitted similarly.

The structure of this embodiment can be freely combined with thestructure of the embodiment modes 1 to 6 and the embodiments 1 to 6. Itis effective to use the EL display panel having the pixel structure ofthis embodiment as a display portion of an electronic equipment of theembodiment 10.

Embodiment 15

For a liquid crystal display device shown in the embodiment 7, variousliquid crystals other than a nematic liquid crystal can be used. Forexample, it is possible to use a liquid crystal disclosed in 1998, SID,“Characteristics and Driving Scheme of Polymer-Stabilized MonostableFLCD Exhibiting Fast Response Time and High Contrast Ratio withGray-Scale Capability” by H. Fume et al.; 1997, SID DIGEST, 841, “AFull-Color Thresholdless Antiferroelectric LCD Exhibiting Wide ViewingAngle with Fast Response Time” by T. Yoshida et al.; 1996, J. Mater.Chem. 6(4), 671-673, “Thresholdless antiferroelectricity in liquidcrystals and its application to displays” by S. Inui et al.; or U.S.Pat. No. 5,594,569.

FIG. 32 shows electro-optical characteristics of single stableferroelectric liquid crystal (FLC) in which the ferroelectric liquidcrystal (FLC) exhibiting a transition series of isometricphase-cholesteric phase-chiral smectic C phase is used, transition ofcholesteric phase-chiral smectic C phase is caused while applying a DCvoltage, and a cone edge is made to almost coincide with a rubbingdirection. A display mode by the ferroelectric liquid crystal as shownin FIG. 32 is called a “Half-V-shaped switching mode.” The vertical axisof the graph shown in FIG. 32 indicates transmissivity (in an arbitraryunit) and the horizontal axis indicates applied voltage. The details ofthe “Half-V-shaped switching mode” is described in “Half-V-shapedswitching mode FLCD” by Terada et al., Collection of Preliminary Papersfor 46th Applied Physics Concerned Joint Lecture Meeting, March 1999, p.1316, and “Time-division full-color LCD with ferroelectric liquidcrystal” by Yoshihara et al., Liquid Crystal, Vol. 3, No. 3, p. 190.

As shown in FIG. 32, it is understood that when such a ferroelectricmixed liquid crystal is used, low voltage driving and gradation displaybecomes possible. For the liquid crystal display device of the presentinvention, it is also possible to use the ferroelectric liquid crystalexhibiting such electro-optical characteristics.

Also, a liquid crystal exhibiting antiferroelectricity in sometemperature range is called an antiferroelectric liquid crystal (AFLC).In mixed liquid crystals including the antiferroelectric liquid crystal,there is one called a thresholdless antiferroelectric mixed liquidcrystal exhibiting electro-optical response characteristics in whichtransmittance is continuously changed with respect to an electric field.Some thresholdless antiferroelectric mixed liquid crystal exhibits theso-called V-shaped electro-optical response characteristics, and aliquid crystal in which its driving voltage is about ±2.5 V (cellthickness is about 1 μm to 2 μm) has also been found.

In general, the thresholdless antiferroelectric mixed liquid crystal haslarge spontaneous polarization, and the dielectric constant of theliquid crystal itself is high. Thus, in the case where the thresholdlessantiferroelectric mixed liquid crystal is used for a liquid crystaldisplay device, it becomes necessary to provide relatively large holdingcapacitance for a pixel. Thus, it is preferable to use the thresholdlessantiferroelectric mixed liquid crystal having small spontaneouspolarization.

Since low voltage driving can be realized by using such a thresholdlessantiferroelectric mixed liquid crystal for the liquid crystal displaydevice of the present invention, low power consumption can be realized.

As described above, the present invention has the following effects.

By carrying out the present invention, it was possible to obtain astable crystalline TFT operation. As a result, it was possible to raisereliability of a semiconductor device including a CMOS circuitfabricated with the crystalline TFT, specifically a pixel region of aliquid crystal display device and a driver circuit provided at theperiphery, so that it was possible to obtain a liquid crystal displaydevice capable of withstanding a long use.

Moreover, according to the present invention, in a second impurityregion formed between a channel forming region of an n-channel TFT and adrain region, it is possible to easily control the individual lengths ofa region (GOLD region) where the second impurity region overlaps with agate electrode and a region (LDD region) not overlapping with the gateelectrode. Specifically, in accordance with a driving voltage of a TFT,it is also possible to determine the respective lengths of the region(GOLD region) where the second impurity region overlaps with the gateelectrode and the region (LDD region) not overlapping with the gateelectrode, which enabled fabrication of TFTs corresponding to therespective driving voltages through the same step in the case where theTFT operations are made by different driving voltages in the samesubstrate.

Further, by causing a part of a gate wiring line and a gate bus line tohave a clad structure, in a large area integrated circuit typified by anactive matrix type liquid crystal display device or an image sensor, theinvention was extremely effective in realization of improvement ofintegration of a circuit.

1. A method of fabricating a semiconductor device, comprising the stepsof: forming a semiconductor layer over a substrate having an insulatingsurface; forming a gate insulating film being in contact with thesemiconductor layer; forming a first conductive layer being in contactwith the gate insulating film; selectively adding an impurity element ofone conductivity type to the semiconductor layer; forming a thirdconductive layer being in contact with the first conductive layer;forming a second conductive layer being in contact with the firstconductive layer and the third conductive layer; forming a gateelectrode from the first conductive layer and the second conductivelayer; forming a gate wiring line from the first conductive layer, thesecond conductive layer, and the third conductive layer; and forming afirst impurity region by selectively adding an impurity element of theone conductivity type to the semiconductor layer.
 2. A method accordingto claim 1, wherein the first conductive layer and the second conductivelayer comprise one kind or plural kinds of elements selected from thegroup consisting of titanium, tantalum, tungsten, and molybdenum, or acompound containing the element as its main material.
 3. A methodaccording to claim 1, wherein the third conductive layer comprises akind of or plural kinds of elements selected from the group consistingof aluminum and copper, or a compound containing the element as its mainmaterial.
 4. A method according to claim 1, wherein the semiconductordevice is one selected from the group consisting of a liquid crystaldisplay device, an EL display device, and an image sensor.
 5. A methodaccording to claim 1, wherein the semiconductor device is one selectedfrom the group consisting of a cellular phone, a video camera, a digitalcamera, a projector, a goggle display, a personal computer, a DVDplayer, an electronic book, and a portable information terminal.
 6. Amethod of fabricating a semiconductor device, comprising the steps of:forming a semiconductor layer over a substrate having an insulatingsurface; forming a gate insulating film being in contact with thesemiconductor layer; forming a first conductive layer being in contactwith the gate insulating film; selectively adding an impurity element ofone conductivity type to the semiconductor layer; forming a thirdconductive layer being in contact with the first conductive layer;forming a second conductive layer being in contact with the firstconductive layer and the third conductive layer; forming a gateelectrode from the first conductive layer and the second conductivelayer; forming a gate wiring line from the first conductive layer, thesecond conductive layer, and the third conductive layer; forming a firstimpurity region by selectively adding an impurity element of the oneconductivity type to the semiconductor layer; and removing a part of thegate electrode.
 7. A method according to claim 6, wherein the firstconductive layer and the second conductive layer comprise one kind orplural kinds of elements selected from the group consisting of titanium,tantalum, tungsten, and molybdenum, or a compound containing the elementas its main material.
 8. A method according to claim 6, wherein thethird conductive layer comprises a kind of or plural kinds of elementsselected from the group consisting of aluminum and copper, or a compoundcontaining the element as its main material.
 9. A method according toclaim 6, wherein the semiconductor device is one selected from the groupconsisting of a liquid crystal display device, an EL display device, andan image sensor.
 10. A method according to claim 6, wherein thesemiconductor device is one selected from the group consisting of acellular phone, a video camera, a digital camera, a projector, a goggledisplay, a personal computer, a DVD player, an electronic book, and aportable information terminal.
 11. A method of fabricating asemiconductor device, comprising the steps of: forming a semiconductorlayer over a substrate having an insulating surface; forming at least afirst island-like semiconductor layer and a second island-likesemiconductor layer by removing a part of the semiconductor layer;forming a gate insulating film being in contact with the firstisland-like semiconductor layer and the second island-like semiconductorlayer; forming a first conductive layer being in contact with the gateinsulating film; forming a second impurity region by adding an impurityelement of one conductivity type to at least a first selected region ofthe first island-like semiconductor layer; forming a third conductivelayer being in contact with the first conductive layer; forming a secondconductive layer being in contact with the first conductive layer andthe third conductive layer; forming a gate electrode from the firstconductive layer and the second conductive layer; forming a gate wiringline from the first conductive layer, the second conductive layer, andthe third conductive layer; forming a first impurity region by adding animpurity element of the one conductivity type to a second selectedregion of the first island-like semiconductor layer; and forming a thirdimpurity region by adding an impurity element of a conductivity typeopposite to the one conductivity type to a selected region of the secondisland-like semiconductor layer.
 12. A method according to claim 11,wherein the first island-like semiconductor layer and the secondisland-like semiconductor layer are formed in one pixel region.
 13. Amethod according to claim 11, wherein the first conductive layer and thesecond conductive layer comprise one kind or plural kinds of elementsselected from the group consisting of titanium, tantalum, tungsten, andmolybdenum, or a compound containing the element as its main material.14. A method according to claim 11, wherein the third conductive layercomprises a kind of or plural kinds of elements selected from the groupconsisting of aluminum and copper, or a compound containing the elementas its main material.
 15. A method according to claim 11, wherein thesemiconductor device is one selected from the group consisting of aliquid crystal display device, an EL display device, and an imagesensor.
 16. A method according to claim 11, wherein the semiconductordevice is one selected from the group consisting of a cellular phone, avideo camera, a digital camera, a projector, a goggle display, apersonal computer, a DVD player, an electronic book, and a portableinformation terminal.
 17. A method of fabricating a semiconductordevice, comprising the steps of: forming a semiconductor layer over asubstrate having an insulating surface; forming at least a firstisland-like semiconductor layer and a second island-like semiconductorlayer by removing a part of the semiconductor layer; forming a gateinsulating film being in contact with the first island-likesemiconductor layer and the second island-like semiconductor layer;forming a first conductive layer being in contact with the gateinsulating film; forming a second impurity region by adding an impurityelement of one conductivity type to at least a first selected region ofthe first island-like semiconductor layer; forming a third conductivelayer being in contact with the first conductive layer; forming a secondconductive layer being in contact with the first conductive layer andthe third conductive layer; forming a gate electrode from the firstconductive layer and the second conductive layer; forming a gate wiringline from the first conductive layer, the second conductive layer, andthe third conductive layer; forming a first impurity region by adding animpurity element of the one conductivity type to a second selectedregion of the first island-like semiconductor layer; forming a thirdimpurity region by adding an impurity element of a conductivity typeopposite to the one conductivity type to a selected region of the secondisland-like semiconductor layer; and removing a part of the gateelectrode.
 18. A method according to claim 17, wherein the firstisland-like semiconductor layer and the second island-like semiconductorlayer are formed in one pixel region.
 19. A method according to claim17, wherein the first conductive layer and the second conductive layercomprise one kind or plural kinds of elements selected from the groupconsisting of titanium, tantalum, tungsten, and molybdenum, or acompound containing the element as its main material.
 20. A methodaccording to claim 17, wherein the third conductive layer comprises akind of or plural kinds of elements selected from the group consistingof aluminum and copper, or a compound containing the element as its mainmaterial.
 21. A method according to claim 17, wherein the semiconductordevice is one selected from the group consisting of a liquid crystaldisplay device, an EL display device, and an image sensor.
 22. A methodaccording to claim 17, wherein the semiconductor device is one selectedfrom the group consisting of a cellular phone, a video camera, a digitalcamera, a projector, a goggle display, a personal computer, a DVDplayer, an electronic book, and a portable information terminal.
 23. Amethod of fabricating a semiconductor device, comprising the steps of:forming a semiconductor layer over a substrate having an insulatingsurface; forming at least a first island-like semiconductor layer and asecond island-like semiconductor layer by removing a part of thesemiconductor layer; forming a gate insulating film being in contactwith the first island-like semiconductor layer and the secondisland-like semiconductor layer; forming a first conductive layer beingin contact with the gate insulating film; forming a second impurityregion by adding an impurity element of one conductivity type to atleast a first selected region of the first island-like semiconductorlayer; forming a third impurity region by adding an impurity element ofa conductivity type opposite to the one conductivity type to a selectedregion of the second island-like semiconductor layer; forming a thirdconductive layer being in contact with the first conductive layer;forming a second conductive layer being in contact with the firstconductive layer and the third conductive layer; forming a gateelectrode from the first conductive layer and the second conductivelayer; forming a gate wiring line from the first conductive layer, thesecond conductive layer, and the third conductive layer; and forming afirst impurity region by adding an impurity element of the oneconductivity type to a second selected region of the first island-likesemiconductor layer.
 24. A method according to claim 23, wherein thefirst island-like semiconductor layer and the second island-likesemiconductor layer are formed in one pixel region.
 25. A methodaccording to claim 23, wherein the first conductive layer and the secondconductive layer comprise one kind or plural kinds of elements selectedfrom the group consisting of titanium, tantalum, tungsten, andmolybdenum, or a compound containing the element as its main material.26. A method according to claim 23, wherein the third conductive layercomprises a kind of or plural kinds of elements selected from the groupconsisting of aluminum and copper, or a compound containing the elementas its main material.
 27. A method according to claim 23, wherein thesemiconductor device is one selected from the group consisting of aliquid crystal display device, an EL display device, and an imagesensor.
 28. A method according to claim 23, wherein the semiconductordevice is one selected from the group consisting of a cellular phone, avideo camera, a digital camera, a projector, a goggle display, apersonal computer, a DVD player, an electronic book, and a portableinformation terminal.